Obstetric Imaging

Obstetric imaging refers to imaging of the female reproductive tract and developing fetus during pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care. Ultrasonography is the 1st-line imaging modality during pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care as it does not emit radiation; thus, it is the safest option for the developing fetus. Obstetricians depend heavily on ultrasound for the detection, monitoring, and assessment of several maternal and fetal conditions without radiation exposure. For example, congenital fetal anomalies, abnormal placentation, poor fetal growth, and abnormal fluid volumes can all be thoroughly assessed using ultrasound. Radiation-emitting imaging modalities (X-ray, CT) are typically reserved for nonobstetric emergency situations.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Overview

Introduction

Obstetric imaging refers to imaging of the female reproductive tract and developing fetus during pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care.

  • Except in rare cases, obstetric imaging is performed via ultrasound.
  • Ultrasound has the following advantages:
    • No radiation exposure
    • Ability to view real-time images of the moving fetus
    • Relatively low cost and wide availability
  • In rare circumstances, obstetric imaging can be obtained via other modalities (e.g., MRI).
  • Nonobstetric imaging in pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care should be ordered judiciously to avoid unnecessary radiation exposure to the developing fetus.

Types of obstetric imaging exams

There are 2 primary types of obstetric imaging exams:

  • Abdominal obstetric ultrasound: good for assessing the fetus, placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity, fluid, and uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall starting in the late 1st trimester through delivery
  • Transvaginal ultrasound:
    • Good for assessing the cervix Cervix The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Externally, the cervix is lined by stratified squamous cells; however, the cervical canal is lined by columnar epithelium. Posterior Abdominal Wall throughout pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care
    • Better for evaluating the fetus and uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall in early pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care

Specific studies

Several specific types of studies can be performed. All are ultrasound exams and may be either abdominal or transvaginal.

  • Dating scan: for the measurement of either the gestational sac or the fetus itself to help establish the gestational age and calculate the estimated date of delivery (EDD)
  • Anatomy survey: to assess the anatomy of both the fetus and the mother
  • Growth scan: Specific measurements are used to calculate the estimated weight of the fetus.
  • Position assessment: to determine the direction in which the fetus is facing within the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall in preparation for delivery
  • Fluid assessments: measurements to help estimate the amount of amniotic fluid
  • Biophysical profile:
    • Assessment of fetal well-being
    • Determination of different types of fetal movements combined with a fluid assessment
  • Doppler studies:
    • Evaluation of the pulse waveforms in specific fetal arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries to assess fetal well-being
    • May demonstrate signs of fetal anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview or uteroplacental insufficiency

Indications for Obstetric Imaging

Routine prenatal care Prenatal care Prenatal care is a systematic and periodic assessment of pregnant women during gestation to assure the best health outcome for the mother and her fetus. Prenatal care prevents and identifies maternal and fetal problems that adversely affect the pregnancy outcome. Prenatal Care

Obstetric imaging is part of routine prenatal care Prenatal care Prenatal care is a systematic and periodic assessment of pregnant women during gestation to assure the best health outcome for the mother and her fetus. Prenatal care prevents and identifies maternal and fetal problems that adversely affect the pregnancy outcome. Prenatal Care, including:

  • 1st trimester:
    • Pregnancy confirmation
    • Pregnancy dating/establishing the EDD
    • Determining the number of fetuses
    • Determining chorionicity of multiple gestations (e.g., monochorionic diamniotic twins)
    • A component of fetal aneuploidy screening
    • Looking for abnormalities of the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall and/or ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries
  • 2nd trimester:
    • Fetal anatomic survey (screening for congenital anomalies)
    • Assessing placentation:
      • Location of the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity (confirming it is away from the internal cervical os)
      • Looking for signs of placental invasion into the myometrium (e.g., placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity accreta spectrum (PAS))
    • Assessing the cervical length
  • 3rd trimester:
    • Estimating fluid volumes
    • Determining fetal position prior to delivery

Monitoring higher-risk pregnancies

  • Assessing fetal growth (growth scans)
  • Following the development of congenital anomalies
  • Assessing fetal status in higher-risk pregnancies (e.g., biophysical profile or doppler studies in an individual with known preeclampsia)
  • Following fetuses at high risk for developing hydrops fetalis

Emergency care

Individuals may present with a number of symptoms in pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care that warrant ultrasound evaluation:

  • Bleeding and/or pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain in early pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care:
    • Rule out ectopic and molar pregnancies.
    • Assess fetal viability/evaluate for potential fetal loss (i.e., spontaneous abortion Spontaneous abortion Spontaneous abortion, also known as miscarriage, is the loss of a pregnancy before 20 weeks' gestation. However, the layperson use of the term "abortion" is often intended to refer to induced termination of a pregnancy, whereas "miscarriage" is preferred for spontaneous loss. Spontaneous Abortion).
  • Bleeding in later pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care: Look for signs of placental abruption (premature separation of the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity).
  • Preterm contractions or pelvic pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain:
    • Cervical length measurement to assess for cervical insufficiency or signs of cervical change
    • Growth scan: important to help the pediatrics team prepare for delivery and provide appropriate counseling to parents (especially in anticipated cases of very premature delivery)
  • Loss of fluid: Assess fluid levels.
  • Decreased fetal movement: biophysical profile for the assessment of fetal movement

During procedures

Ultrasound is often used to assist the physician during procedures such as:

  • Amniocentesis
  • Chorionic villus sampling
  • Cordocentesis
  • External cephalic version

Performing an Obstetric Ultrasound

Ultrasound exam technique

  • Positioning of the individual:
    • Abdominal scans:
      • Supine with access to the lower abdomen
      • 2nd/3rd trimesters: A hip roll (rolled-up sheet) should be placed under 1 of the hips of the individual so that they are not lying flat on their back.
    • Transvaginal scans: lower lithotomy
  • Tips for obtaining good images:
    • Maximize contact between the individual’s skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin and ultrasound probe.
    • Use plenty of ultrasound gel.
  • Depth and gain:
    • Determines the field of view and echogenicity characteristics of the tissue
    • In early pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care, the entire gestational sac should be viewed at once.

Components of the exam

Components that should be assessed during all 2nd and 3rd trimester obstetric ultrasounds:

  • Fetal heart rate
  • Fetal position: What part of the fetus is in the lower uterine segment (and thus will deliver 1st)?
  • Amniotic fluid volume
  • Placental location in relation to the cervix Cervix The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Externally, the cervix is lined by stratified squamous cells; however, the cervical canal is lined by columnar epithelium. Posterior Abdominal Wall
  • Any other objective of the study (e.g., anatomic survey, growth assessment, etc.)

Preparing for general image interpretation

Prior to interpretation of any image, the physician should take certain preparatory steps. The same systematic approach should be followed every time.

  • Confirm name, date, and time on all images.
  • Review the individual’s medical history and physical examination findings.
  • Confirm that the appropriate exams and techniques that can best assess the suspected pathology were ordered/performed.
  • Determine orientation of the image.
  • Have any other previously obtained relevant images available for comparison handy.

Normal Findings on Obstetric Ultrasound

Pregnancy confirmation

  • Gestational sac:
    • Earliest sign of intrauterine pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care, seen around 4.5‒5 weeks gestational age (wga)
    • Should be visible in the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall if the quantitative serum β-hCG is > 2,000
    • Anechoic, well-defined round structure
    • Surrounded by an echogenic rim, representing the decidual reaction
  • Yolk sac:
    • Hyperechoic ring-like structure within the gestational sac
    • 1st seen at approximately 5‒6 wga and disappears at approximately 10 wga
  • Fetal pole:
    • The fetus itself
    • Visible around 5.5‒6 wga
    • A heartbeat is usually visible as soon as the fetal pole is visible.
  • Corpus luteum cyst:
    • An adnexal mass representing the follicle from which the oocyte ovulated, which persists throughout the 1st trimester of pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care
    • Produces progesterone, which is vital for survival of the pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care
    • Sonographic appearance:
      • Cyst may be simple or complex.
      • Typically surrounded by ↑ vasculature, seen on Doppler studies as a circumferential rim of color known as the “ring of fire”

Determining viability

Establishing that a pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care is viable requires:

  • Intrauterine location:
    • Should be within the main uterine body endometrium
    • At least a gestational sac and yolk sac must be seen in order to establish the pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care location (a gestational sac alone is not enough).
  • A detectable fetal heart rate, usually between about 120‒160 per minute (may be slightly higher at certain points in early pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care)
Calculation of the fetal heart rate by ultrasound

Calculation of the fetal heart rate by ultrasound


Image by Hetal Verma.

Pregnancy dating via ultrasound

  • 1st trimester obstetric ultrasound is the best and the most accurate tool to estimate gestational age and calculate the EDD.
    • Most accurate in the 1st trimester
    • Ultrasound gets less and less accurate as gestation progresses due to normal genetic variations (e.g., height of parents) and due to effects of the intrauterine environment (e.g., smokers have worse placental perfusion).
  • 1st-trimester dating:
    • Measure the crown-rump length.
    • Crown-rump length should be consistent with the expected gestational age based on the last menstrual period (LMP).
  • 2nd and 3rd trimester dating:
    • Performed using fetal growth scan, which can be used to calculate the estimated fetal weight (EFW) and EDD from specific measurements
    • Measurements include:
      • Biparietal diameter and head circumference
      • Abdominal diameter and circumference
      • Femur length

Determining the number of embryos

  • The uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall should be fully evaluated in all planes to get an accurate fetal count.
  • Multiple gestation: when > 1 fetus is present
    • Twins
    • Higher-order multiples (e.g., triplets, quadruplets, etc.)

Determining chorionicity in multiple gestations

Chorionicity describes whether the fetuses share a chorion or amnion. Chorionicity can be established by different ultrasound findings in different types of twins:

  • Dichorionic/diamniotic twins (each twin is in their own chorioamnion and has their own placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity): 
    • Thick intertwin membrane
    • Lambda sign: a thick, triangular protrusion of tissue leading up to the intertwin membrane
    • 2 separate placentas (however, if they are right next to each other, they may appear as a single placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity)
  • Monochorionic/diamniotic (twins are in their own amniotic sac, but share a chorion and placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity):
    • Thin intertwin membrane
    • T sign: The intertwin membrane comes straight into the sac wall, without the thick triangular protrusion of tissue that is seen in dichorionic diamniotic twins.
    • Single placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity
  • Monochorionic/monoamniotic (twins share a chorioamnion and placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity)
    • No intertwin membrane
    • Single placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity

Anatomic survey

A complete anatomic survey assesses both the maternal reproductive tract and looks for fetal anomalies. Some of the important features evaluated include:

  • Maternal anatomy:
    • Cervical length: should be > 25 mm until at least 24 wga
    • Presence of any uterine fibroids distorting the cavity, or in the lower uterine segment, which may be in the way of a potential cesarean incision
    • Adnexal masses
  • Placental and umbilical cord assessment:
    • Placenta:
      • Should not cover the internal cervical os
      • Should not invade into the underlying myometrium
    • Umbilical cord:
      • Should have 3 visible vessels
      • Should insert near the middle of the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity and at the fetal umbilicus
      • Vessels should be surrounded by protective jelly all the way down to the placental insertion.
  • Fetal anatomy: Multiple structures, including all major organs, are measured and assessed.
    • Brain 
    • Face
    • Heart/ lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, including 4-chamber and outflow-tract views of the heart
    • Abdomen
    • Limbs
    • Genitalia

Amniotic fluid assessment

Amniotic fluid can be assessed in 2 ways:

  • Single deepest pocket (SDP):
    • Measures the single deepest vertical pocket of fluid 
    • The measured pocket must be free of the umbilical cord and fetal parts.
    • Normal range (2nd and 3rd trimesters): 2‒8 cm
  • Amniotic fluid index (AFI):
    • Divide the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall into 4 quadrants and obtain an SDP for each quadrant; the AFI is the sum of the 4 SDP measurements.
    • Normal range (2nd and 3rd trimesters): 5‒24 cm

Summary of normal findings on obstetric ultrasound

  • Single intrauterine pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care
  • Fetal heart rate between 120 and 160 per minute
  • No significant congenital anomalies identified
  • Normal placental attachment, away from the cervical os
  • 3-vessel umbilical cord
  • Normal volume of amniotic fluid
  • Cervical length > 25 mm until at least 24 wga
  • Appropriate fetal weight for gestational age
  • Vertex fetal positioning in the late 3rd trimester (not important earlier)

Abnormal Findings on Obstetric Ultrasound

Numerous abnormalities can be identified on obstetric ultrasound.

Abnormal/nonviable pregnancies

Threatened and missed abortions:

  • Threatened abortion:
    • A pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care with clinical signs indicating the possibility of a miscarriage (e.g., bleeding and cramping)
    • Fetal heart rate (FHR) will still be present.
    • Hyper- or hypo-echoic areas may be visible near the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity or behind the membranes, suggestive of bleeding.
  • Missed abortion: 
    • A fetus is present in the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall, but no longer viable.
    • FHR will be absent.

Ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy:

An ectopic pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care is characterized by implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week outside the uterine cavity. Ultrasound findings include:

  • Heterogeneous adnexal mass
  • Tubal ring sign: an echogenic ring separating the ectopic pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care from the ovary 
  • Pseudogestational sac:
    • Cystic sac within the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall, with no embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week
    • Decidual reaction present: thickened echogenic endometrium surrounding the intrauterine sac (because pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview are still being produced by the ectopic pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care)
    • Misleading, because it can appear identical to an early gestational sac before the yolk sac appears
  • No identifiable pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care when the HCG is > 2,000
  • Free peritoneal fluid, possibly with low-level internal echos suggests hemorrhage from ruptured ectopic pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care.
  • Note on heterotopic pregnancies (twin gestations with 1 fetus in the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall and 1 ectopic):
    • Possible, but exceedingly rare
    • If an intrauterine gestation is identified, the adnexa should still be evaluated for masses; if it is not seen, heterotopic pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care can be excluded.
Transvaginal ultrasound

Transvaginal ultrasound showing an empty uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall (left) with endometrial thickening and an echogenic mass (right) representing an ectopic pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care adjacent to normal ovarian tissue

Image by Hetal Verma.

Molar pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care:

Molar pregnancies are a type of gestational trophoblastic disease Gestational trophoblastic disease Gestational trophoblastic diseases are a spectrum of placental disorders resulting from abnormal placental trophoblastic growth. These disorders range from benign molar pregnancies (complete and partial moles) to neoplastic conditions such as invasive moles and choriocarcinoma. Gestational Trophoblastic Disease that occur due to abnormal fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week.

  • How they occur:
    • Complete mole: An enucleated ovum (i.e., an egg without any DNA DNA The molecule DNA is the repository of heritable genetic information. In humans, DNA is contained in 23 chromosome pairs within the nucleus. The molecule provides the basic template for replication of genetic information, RNA transcription, and protein biosynthesis to promote cellular function and survival. DNA Types and Structure) is fertilized by 2 sperm (complete mole).
    • Partial mole: 2 sperm fertilize a haploid ovum.
  • Ultrasound findings:
    • Enlarged uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall
    • Heterogeneous tissue within the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall with a classic “snowstorm” appearance
      • Cystic spaces: anechoic
      • Placental tissue: hyperechoic
    • Fetus/fetal parts may or may not be present.
    • Large bilateral ovarian cysts Ovarian cysts Ovarian cysts are defined as collections of fluid or semiliquid material, often walled off by a membrane, located in the ovary. These cysts are broadly categorized as either functional or neoplastic. Neoplastic ovarian cysts are subcategorized as either benign or malignant. Ovarian Cysts may be present.
Transvaginal ultrasonography molar pregnancy obstertic imaging

Transvaginal ultrasonography showing a molar pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care: The pattern is described as a “cluster of grapes.”

Image: “Molar pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care” by Mikael Häggström. License: CC0 1.0

Retained products of conception:

After an abortion (either spontaneous or induced), or postpartum after delivery of the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity, tissue may be retained within the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall. This phenomenon is known as retained products of conception and can lead to hemorrhage and infection. Ultrasound findings include:

  • Intrauterine, heterogeneous material (typically hyperechoic)
  • Enlarged uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall
  • Increased blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure to the mass on Doppler mode
Transvaginal ultrasound demonstrates a heterogenous echogenic mass

A transvaginal ultrasound demonstrates a heterogeneous echogenic mass in the endometrial cavity (black arrow), representing retained products of conception.

Image: “Gray-scale US demonstrates an echogenic mass in the endometrial cavity (black arrow)” by Maureen P. Kohi et al. License: CC BY 3.0

Fetal abnormalities

Nuchal translucency for aneuploidy screening:

An assessment of the nuchal translucency (or thickness of the nuchal fold at the back of the neck) is a part of common aneuploidy screening tests.

  • Measures the hypoechoic region between the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin and soft tissue behind the cervical spine
  • A thickened nuchal fold increases the risk for:
    • Trisomy 21 (most common)
    • Trisomies 13 and 18
    • Turner syndrome Turner syndrome Turner syndrome is a genetic condition affecting women, in which 1 X chromosome is partly or completely missing. The classic result is the karyotype 45,XO with a female phenotype. Turner syndrome is associated with decreased sex hormone levels and is the most common cause of primary amenorrhea. Turner Syndrome
    • Major congenital heart disease
    • Hydrocephalus
  • > 100 different developmental and genetic syndromes have also been associated with an increased nuchal fold
Nuchal translucency measurements

Nuchal translucency (NT) measurements:
Figure (A) shows a normal fetus (looking up). Figure (B) shows a fetus with trisomy 21 (looking down), demonstrating increased NT thickness.

Image: “Nuchal translucency measurements” by Barati M et al. License: CC BY 2.5

Congenital anomalies:

Almost any area of the body can develop incorrectly, leading to congenital anomalies. Many of them are visible on ultrasound. Some of the clinically important anomalies and their associated ultrasound findings include:

  • Cardiac defects (most common, found in approximately 1% of births):
    • A full fetal echo can be performed in utero → full spectrum of lesions can be identified
    • Clinically important defects include:
      • Tetralogy of Fallot Tetralogy of Fallot Tetralogy of Fallot is the most common cyanotic congenital heart disease. The disease is the confluence of 4 pathologic cardiac features: overriding aorta, ventricular septal defect, right ventricular outflow obstruction, and right ventricular hypertrophy. Tetralogy of Fallot
      • Transposition of the great vessels Transposition of the Great Vessels Transposition of the great vessels (TGV) is a cyanotic congenital heart disease characterized by "switching" of the great arteries. There are 2 presentations: the dextro (D)- and levo (L)-looped forms. The L-looped form is rare and congenitally corrected, as the ventricles are also switched. Transposition of the Great Vessels
      • Truncus arteriosus Truncus arteriosus Truncus arteriosus (TA) is a congenital heart defect characterized by the persistence of a common cardiac arterial trunk tract that fails to divide into the pulmonary artery and aorta during embryonic development. Truncus arteriosus is a rare congenital malformation with a high mortality rate within the 1st 5 weeks of life if not managed promptly. Truncus Arteriosus
      • Holes: ventral septal defect, atrioventricular canal defect
      • Valve defects: stenosis, regurgitation, atresia
      • Coarctation of the aorta Coarctation of the aorta Coarctation of the aorta is a narrowing of the aorta between the aortic arch and the iliac bifurcation commonly around the point of insertion of the ductus arteriosus. Coarctation of the aorta is typically congenital and the clinical presentation depends on the age of the patient. Coarctation of the Aorta
  • Neural tube defects Neural tube defects Neural tube defects (NTDs) are the 2nd-most common type of congenital birth defects. Neural tube defects can range from asymptomatic (closed NTD) to very severe malformations of the spine or brain (open NTD). Neural tube defects are caused by the failure of the neural tube to close properly during the 3rd and 4th week of embryological development. Neural Tube Defects (2nd most common):
    • Anencephaly (most common neural tube defect): absence of the brain
    • Cephalocele: cranial defects through which the brain or meninges Meninges The brain and the spinal cord are enveloped by 3 overlapping layers of connective tissue called the meninges. The layers are, from the most external layer to the most internal layer, the dura mater, arachnoid mater, and pia mater. Between these layers are 3 potential spaces called the epidural, subdural, and subarachnoid spaces. Meninges herniate outside the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull
    • Spina bifida/meningocele/myelomeningocele: protrusion of the spinal contents through bony defects in the spine
  • Abdominal wall defects:
    • Omphalocele Omphalocele Omphalocele is a congenital anterior abdominal wall defect in which the intestines are covered by peritoneum and amniotic membranes. The condition results from the failure of the midgut to return to the abdominal cavity by 10 weeks' gestation. Omphalocele: Multiple bowel loops (+/- liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver) are seen herniating through a membrane-covered midline abdominal defect.
    • Gastroschisis Gastroschisis Gastroschisis is a congenital abdominal wall defect characterized by the complete lack of closure of the abdominal musculature. A portion of intestine does not return to the abdominal cavity, thereby remaining in its early embryonic herniated state but with no coverings. Gastroschisis: Bowel loops protrude outside the abdominal cavity without an overlying membrane, through a lateral abdominal wall defect.

Intrauterine growth restriction (IUGR):

  • Abnormally low EFW on a growth scan
  • Typically defined as an EFW < 10th percentile for the estimated gestational age

Hydrops fetalis:

  • Hydrops fetalis refers to abnormal fluid collections in ≥ 2 of the following fetal compartments:
    • Significant skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema (present in almost all hydropic infants) > 5 mm
    • Ascites Ascites Ascites is the pathologic accumulation of fluid within the peritoneal cavity that occurs due to an osmotic and/or hydrostatic pressure imbalance secondary to portal hypertension (cirrhosis, heart failure) or non-portal hypertension (hypoalbuminemia, malignancy, infection). Ascites
    • Pleural effusions
    • Pericardial effusions
  • Other potential ultrasound findings:
    • Polyhydramnios Polyhydramnios Polyhydramnios is a pathological excess of amniotic fluid. Common causes of polyhydramnios include fetal anomalies, gestational diabetes, multiple gestations, and congenital infections. Patients are often asymptomatic but may present with dyspnea, extremity swelling, or abdominal distention. Polyhydramnios
    • Increased nuchal translucency
    • Increased placental thickness
Ultrasound images of an infant with hydrops fetalis

Ultrasound images of an infant with hydrops fetalis:
Axial (A) and oblique sagittal (B) images showing fetal ascites (short white arrow) and floating bowel loops (b); axial image of the fetal head (C) showing significant scalp edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema (long white arrow)

Image: “Ultrasound images of an infant with hydrops fetalis” by Afzal et al. License: CC BY 3.0

Placental abnormalities Placental abnormalities Normal placental structure and function are essential for a healthy pregnancy. Some of the most common placental abnormalities include structural anomalies (such as a succenturiate lobe or velamentous cord insertion), implantation anomalies (such as placenta accreta and placenta previa), and functional anomalies (such as placental insufficiency). Placental Abnormalities

Abnormal placentation:

Abnormal placentation refers to abnormal implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week of the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity. Ultrasound findings may show an abnormal placental location, or show it invading into the uterine wall.

  • Placenta previa: Placenta covers the internal cervical os.
  • Low-lying placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity: Placenta is within 2 cm of the internal cervical os.
  • PAS: Placenta is abnormally adherent to the uterine wall.
    • Placenta accreta (approximately 65%): Placenta attaches directly to the myometrium due to the partial or total absence of the decidua basalis.
    • Placenta increta (15%): Placental villi invade into the myometrium.
    • Placenta percreta (approximately 20%): Placental villi penetrate through the entire myometrium and may invade other surrounding structures.
Types of abnormal placentation

Types of abnormal placentation

Image by Lecturio. License: CC BY-NC-SA 4.0

Placental abruption:

Placental abruption refers to the premature separation of the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity, leading to maternal-fetal hemorrhage. Ultrasound findings are usually only seen in large abruptions and may include:

  • Hyper- or iso-echoic retroplacental hematoma
  • Heterogeneity within the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity
  • Separation of placental edges from the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall
  • Placental thickening
Ultrasound acute placental abruption

Acute placental abruption:
Note the bulky heterogeneous placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity (arrows) in this hypertensive, 29-week gestational age pregnant individual.

Image: “Acute placental abruption” by Kinare A. License: CC BY 2.0

Fluid abnormalities

Fluid assessment, at least with an SDP, should be part of every obstetric ultrasound. Fluid abnormalities include:

  • Polyhydramnios Polyhydramnios Polyhydramnios is a pathological excess of amniotic fluid. Common causes of polyhydramnios include fetal anomalies, gestational diabetes, multiple gestations, and congenital infections. Patients are often asymptomatic but may present with dyspnea, extremity swelling, or abdominal distention. Polyhydramnios: too much fluid (SDP > 8 cm or AFI > 24 cm)
  • Oligohydramnios Oligohydramnios Oligohydramnios refers to amniotic fluid volume less than expected for the current gestational age. Oligohydramnios is diagnosed by ultrasound and defined as an amniotic fluid index (AFI) of ‰¤ 5 cm or a single deep pocket (SDP) of < 2 cm in the 2nd or 3rd trimester. Oligohydramnios: too little fluid (SDP < 2 cm or AFI < 5 cm)
  • Anhydramnios: no fluid (no measurable pockets of fluid)
Fetal environment - measuring single vertical pocket of liquor

Single deepest vertical pocket of fluid is measured using ultrasound to assess amniotic fluid volume. Polyhydramnios Polyhydramnios Polyhydramnios is a pathological excess of amniotic fluid. Common causes of polyhydramnios include fetal anomalies, gestational diabetes, multiple gestations, and congenital infections. Patients are often asymptomatic but may present with dyspnea, extremity swelling, or abdominal distention. Polyhydramnios is present in this case (SDP > 8 cm).

Image: “Demonstration of the technique to measure a single vertical pocket of liquor” by Kinare A. License: CC BY 2.0

Other Imaging Modalities in Pregnancy

Nonsonographic obstetric imaging

Obstetric imaging outside of ultrasound is of limited utility and confined to very specific indications.

  • MRI may be used for:
    • Evaluation of specific fetal congenital abnormalities noted on ultrasound
    • Characterization of maternal pelvic anatomy in cases with unusual or complex abnormalities
  • CT is almost never indicated for evaluation of the fetus or maternal pelvic anatomy for obstetric indications.
Mri findings fetal chiari ii malformation

Magnetic resonance imaging findings in a fetus at 23 wga, suggestive of a fetal Chiari II malformation:
a: T2-weighted sagittal image demonstrating a lumbosacral neural tube defect (encircled) with cerebellar tonsillar herniation (arrow)
b: T2-weighted sagittal image demonstrating a myelomeningocele (encircled) from L2 to the end of the sacrum
c: T2-weighted axial image showing the myelomeningocele (encircled)
d: T2-weighted axial image demonstrating hydrocephalus

Image: “Neural tube defect with tonsillar herniation” by Loomba R. License: CC BY 3.0

Nonobstetric imaging during pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care

  • Ultrasound and MRI are the preferred modalities due to the lack of radiation exposure.
    • Example: Abdominal ultrasound is the preferred initial test for appendicitis Appendicitis Appendicitis is the acute inflammation of the vermiform appendix and the most common abdominal surgical emergency globally. The condition has a lifetime risk of 8%. Characteristic features include periumbilical abdominal pain that migrates to the right lower quadrant, fever, anorexia, nausea, and vomiting. Appendicitis over CT.
  • Chest X-ray:
    • Abdomen should be shielded.
    • Only done when absolutely necessary (e.g., clinical deterioration with concern for pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. Pneumonia).
    • Indicated in cases of choriocarcinoma to evaluate for lung metastases
  • If CT is the modality required to make an important diagnosis (e.g., an individual presenting with stroke symptoms):
    • A single CT scan in pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care is considered relatively safe.
    • Risks/benefits should be carefully weighed and discussed with the individual.
X-ray round left midlung airspace opacity

Round, left midlung airspace opacity in an individual reported to have gestational trophoblastic disease Gestational trophoblastic disease Gestational trophoblastic diseases are a spectrum of placental disorders resulting from abnormal placental trophoblastic growth. These disorders range from benign molar pregnancies (complete and partial moles) to neoplastic conditions such as invasive moles and choriocarcinoma. Gestational Trophoblastic Disease

Image: “The appearance of lung metastasis in patient that was reported” by Ghaemmaghami F. License: CC BY 2.5

Clinical Relevance

Obstetric imaging is a critical part of almost all obstetric care.

Diagnosing abnormal pregnancies

  • Ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy: pregnancies that have implanted outside the uterine cavity, most commonly in the fallopian tubes Fallopian tubes The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The fallopian tubes receive an ovum after ovulation and help move it and/or a fertilized embryo toward the uterus via ciliated cells lining the tubes and peristaltic movements of its smooth muscle. Posterior Abdominal Wall. Ectopic pregnancies are nonviable and may rupture as they grow, leading to life-threatening maternal hemorrhage. Individuals typically present with lower abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain and/or abnormal bleeding; a urine pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care test will be positive. Management is either with methotrexate or surgical excision, depending on the case.
  • Molar pregnancy Pregnancy Pregnancy is the time period between fertilization of an oocyte and delivery of a fetus approximately 9 months later. The 1st sign of pregnancy is typically a missed menstrual period, after which, pregnancy should be confirmed clinically based on a positive β-HCG test (typically a qualitative urine test) and pelvic ultrasound. Pregnancy: Diagnosis, Maternal Physiology, and Routine Care: arises from abnormal fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week, resulting in abnormal embryonic genetics Genetics Genetics is the study of genes and their functions and behaviors. Basic Terms of Genetics. The classic clinical presentation is hyperemesis (severe nausea and vomiting), 1st trimester vaginal bleeding, and enlarged uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall relative to the individual’s estimated gestational age. Diagnosis is by ultrasound and abnormally elevated hCG levels. Suction and curettage are the mainstays of treatment and post-procedural monitoring is essential to ensure that invasive disease does not develop.
Table: Characteristics of complete versus partial molar pregnancies
Complete mole Partial mole
Karyotype 46,XX or 46,XY Triploid (69,XXX, 69,XXY, or 69,XYY)
Formed from Enucleated egg and a single sperm 2 sperm and 1 egg
Fetal parts Absent Present
Human chorionic gonadotropin levels ↑↑↑
Ultrasound findings
  • Honeycomb uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall
  • “Clusters of grapes”
  • “Snowstorm” uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall
Reveals fetal parts
Malignancy risk Higher risk for choriocarcinoma Rare

Fetal issues

  • Fetal anomalies: abnormalities in the fetus due to either genetic, chromosomal, or developmental abnormalities. Fetal anomalies can occur in any organ system throughout the body; clinical presentation and management will depend on the specific anomalies. The most common are cardiac and neural tube defects.
  • Fetal growth restriction Fetal growth restriction Fetal growth restriction (FGR), also known as intrauterine fetal growth restriction (IUGR), is an estimated fetal weight (EFW) or abdominal circumference < 10th percentile for gestational age. The term small for gestational age (SGA) is sometimes erroneously used interchangeably with FGR. Fetal Growth Restriction (FGR): a condition, also known as IUGR, referring to poor fetal growth in utero due to environmental factors. Fetal growth restriction Fetal growth restriction Fetal growth restriction (FGR), also known as intrauterine fetal growth restriction (IUGR), is an estimated fetal weight (EFW) or abdominal circumference < 10th percentile for gestational age. The term small for gestational age (SGA) is sometimes erroneously used interchangeably with FGR. Fetal Growth Restriction is typically defined as an EFW < the 10th percentile based on the estimated gestational age. The primary pathogenesis is due to poor perfusion through the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity, limiting the oxygen and nutrients available to the fetus. There are numerous fetal, maternal, and placental etiologies.
  • Hydrops fetalis: an abnormal collection of fluid within the fetus. Diagnosis requires at least 2 of the following (all visible on obstetric ultrasound) presentations: skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema, ascites, pleural effusions, or pericardial effusions. Hydrops fetalis is often caused by maternal antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins against fetal RBCs leading to severe fetal anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview, although there are several nonimmune etiologies as well (e.g., infection with parvovirus B19 Parvovirus B19 Primate erythroparvovirus 1 (generally referred to as parvovirus B19, B19 virus, or sometimes erythrovirus B19) ranks among the smallest DNA viruses. Parvovirus B19 is of the family Parvoviridae and genus Erythrovirus. In immunocompetent humans, parvovirus B19 classically results in erythema infectiosum (5th disease) or "slapped cheek syndrome." Parvovirus B19). Often, the primary management is close monitoring, with induced preterm delivery when fetal status begins to deteriorate.

Abnormal placentation

  • Placenta previa: the presence of placental tissue covering the internal cervical os. When the cervix Cervix The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Externally, the cervix is lined by stratified squamous cells; however, the cervical canal is lined by columnar epithelium. Posterior Abdominal Wall begins dilating, the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity will become “detached” over the opening cervical os, resulting in life-threatening maternal-fetal hemorrhage. Diagnosis is using obstetric ultrasound (usually transvaginal). Management is by avoiding any digital cervical exams and delivering the infant via cesarean delivery Cesarean Delivery Cesarean delivery (CD) is the operative delivery of ≥ 1 infants through a surgical incision in the maternal abdomen and uterus. Cesarean deliveries may be indicated for a number of either maternal or fetal reasons, most commonly including fetal intolerance to labor, arrest of labor, a history of prior uterine surgery, fetal malpresentation, and placental abnormalities. Cesarean Delivery prior to the onset of labor Labor Labor is the normal physiologic process defined as uterine contractions resulting in dilatation and effacement of the cervix, which culminates in expulsion of the fetus and the products of conception. Normal and Abnormal Labor.
  • PAS: describes a spectrum in which the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity is abnormally and firmly adherent to the uterine wall. In this situation, the placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity is unable to detach postpartum and continues to bleed, often heavily, resulting in life-threatening hemorrhage. The 3 degrees of PAS are placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity accreta, increta, and percreta. Diagnosis is via ultrasound, and management is usually surgical, often including a planned hysterectomy at the time of cesarean delivery Cesarean Delivery Cesarean delivery (CD) is the operative delivery of ≥ 1 infants through a surgical incision in the maternal abdomen and uterus. Cesarean deliveries may be indicated for a number of either maternal or fetal reasons, most commonly including fetal intolerance to labor, arrest of labor, a history of prior uterine surgery, fetal malpresentation, and placental abnormalities. Cesarean Delivery.

Abnormal amniotic fluid volumes

  • Polyhydramnios Polyhydramnios Polyhydramnios is a pathological excess of amniotic fluid. Common causes of polyhydramnios include fetal anomalies, gestational diabetes, multiple gestations, and congenital infections. Patients are often asymptomatic but may present with dyspnea, extremity swelling, or abdominal distention. Polyhydramnios: a pathological excess of amniotic fluid. Common causes include fetal anomalies, gestational diabetes, multiple gestations, and congenital infections. Diagnosis is made based on ultrasound with an AFI ≥ 24 cm or an SDP ≥ 8 cm. Management of moderate-to-severe cases includes amnioreduction, medical therapy, and careful management during labor Labor Labor is the normal physiologic process defined as uterine contractions resulting in dilatation and effacement of the cervix, which culminates in expulsion of the fetus and the products of conception. Normal and Abnormal Labor. Major risks include cord prolapse and placental abruption at the time of membrane rupture.
  • Oligohydramnios Oligohydramnios Oligohydramnios refers to amniotic fluid volume less than expected for the current gestational age. Oligohydramnios is diagnosed by ultrasound and defined as an amniotic fluid index (AFI) of ‰¤ 5 cm or a single deep pocket (SDP) of < 2 cm in the 2nd or 3rd trimester. Oligohydramnios: a condition characterized by pathologically low amniotic fluid volumes. Oligohydramnios Oligohydramnios Oligohydramnios refers to amniotic fluid volume less than expected for the current gestational age. Oligohydramnios is diagnosed by ultrasound and defined as an amniotic fluid index (AFI) of ‰¤ 5 cm or a single deep pocket (SDP) of < 2 cm in the 2nd or 3rd trimester. Oligohydramnios is diagnosed by ultrasound and defined as an AFI of ≤ 5 cm or an SDP of < 2 cm in the 2nd and 3rd trimesters. Etiologies include uteroplacental insufficiency, drugs, fetal malformations (especially those related to renal/urinary systems), and maternal TORCH infections TORCH infections Congenital infections are acquired in utero or during passage through the birth canal at birth and can be associated with significant morbidity and mortality for the infant. The TORCH infections are a group of congenital infections grouped due to their similar presentation. The acronym TORCH arises from the names of the infectious agents that cause the diseases included in this group: toxoplasmosis, other agents (syphilis, varicella zoster virus (VZV), parvovirus B19, and HIV), rubella, CMV, and herpes simplex. Congenital TORCH Infections. Management involves close antenatal monitoring because significant complications are possible, including FGR, preterm delivery, and developmental deformities.

References

  1. Shipp, T.D. (2021). Overview of ultrasound examination in obstetric and gynecology. In Barss, V.A. (Ed.), UpToDate. Retrieved July 30, 2021, from https://www.uptodate.com/contents/overview-of-ultrasound-examination-in-obstetrics-and-gynecology 
  2. Magann, E., Ross, M.G. (2021). Assessment of amniotic fluid volume. In Barss, V.A., (Ed.), UpToDate. Retrieved July 30, 2021, from https://www.uptodate.com/contents/assessment-of-amniotic-fluid-volume 
  3. Berkowitz, R.S., et al. (2020). Hydatidiform mole: Epidemiology, clinical features, and diagnosis. UpToDate. Retrieved July 30, 2021, from https://www.uptodate.com/contents/hydatidiform-mole-epidemiology-clinical-features-and-diagnosis
  4. Weeks, A. (2021). Retained placenta after vaginal birth. UpToDate. Retrieved July 30, 2021, from https://www.uptodate.com/contents/retained-placenta-after-vaginal-birth

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