Obstetric Imaging

Overview

Introduction

Obstetric imaging refers to imaging of the female reproductive tract and developing fetus during pregnancy.

• 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 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, fluid, and uterus starting in the late 1st trimester through delivery
• Transvaginal ultrasound:
  • Good for assessing the cervix throughout pregnancy
  • Better for evaluating the fetus and uterus in early pregnancy

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 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 to assess fetal well-being
  • May demonstrate signs of fetal anemia or uteroplacental insufficiency

Indications for Obstetric Imaging

Routine prenatal care

Obstetric imaging is part of routine 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 and/or ovaries
• 2nd trimester:
  • Fetal anatomic survey (screening for congenital anomalies)
  • Assessing placentation:
    • Location of the placenta (confirming it is away from the internal cervical os)
    • Looking for signs of placental invasion into the myometrium (e.g., placenta 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 that warrant ultrasound evaluation:

• Bleeding and/or pain in early pregnancy:
  • Rule out ectopic and molar pregnancies.
  • Assess fetal viability/evaluate for potential fetal loss (i.e., spontaneous abortion).
• Bleeding in later pregnancy: Look for signs of placental abruption (premature separation of the placenta).
• Preterm contractions or pelvic 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 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, 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
• 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, seen around 4.5‒5 weeks gestational age (wga)
  • Should be visible in the uterus 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
  • Produces progesterone, which is vital for survival of the pregnancy
  • 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 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 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 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 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): 
  • 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)
• Monochorionic/diamniotic (twins are in their own amniotic sac, but share a chorion and placenta):
  • 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
• Monochorionic/monoamniotic (twins share a chorioamnion and placenta)
  • No intertwin membrane
  • Single placenta

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 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, 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 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
• 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 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 or behind the membranes, suggestive of bleeding.
• Missed abortion: 
  • A fetus is present in the uterus, but no longer viable.
  • FHR will be absent.

Ectopic pregnancy:

An ectopic pregnancy is characterized by implantation outside the uterine cavity. Ultrasound findings include:

• Heterogeneous adnexal mass
• Tubal ring sign: an echogenic ring separating the ectopic pregnancy from the ovary 
• Pseudogestational sac:
  • Cystic sac within the uterus, with no embryo
  • Decidual reaction present: thickened echogenic endometrium surrounding the intrauterine sac (because pregnancy hormones are still being produced by the ectopic pregnancy)
  • Misleading, because it can appear identical to an early gestational sac before the yolk sac appears
• No identifiable pregnancy when the HCG is > 2,000
• Free peritoneal fluid, possibly with low-level internal echos suggests hemorrhage from ruptured ectopic pregnancy.
• Note on heterotopic pregnancies (twin gestations with 1 fetus in the uterus 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 can be excluded.

Molar pregnancy:

Molar pregnancies are a type of gestational trophoblastic disease that occur due to abnormal fertilization.

• How they occur:
  • Complete mole: An enucleated ovum (i.e., an egg without any DNA) is fertilized by 2 sperm (complete mole).
  • Partial mole: 2 sperm fertilize a haploid ovum.
• Ultrasound findings:
  • Enlarged uterus
  • Heterogeneous tissue within the uterus with a classic “snowstorm” appearance
    • Cystic spaces: anechoic
    • Placental tissue: hyperechoic
  • Fetus/fetal parts may or may not be present.
  • Large bilateral ovarian cysts may be present.

Retained products of conception:

After an abortion (either spontaneous or induced), or postpartum after delivery of the placenta, tissue may be retained within the uterus. 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
• Increased blood flow to the mass on Doppler mode

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 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
  • Major congenital heart disease
  • Hydrocephalus
• > 100 different developmental and genetic syndromes have also been associated with an increased nuchal fold

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
    • Transposition of the great vessels
    • Truncus arteriosus
    • Holes: ventral septal defect, atrioventricular canal defect
    • Valve defects: stenosis, regurgitation, atresia
    • Coarctation of the aorta
• 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 herniate outside the skull
  • Spina bifida/meningocele/myelomeningocele: protrusion of the spinal contents through bony defects in the spine
• Abdominal wall defects:
  • Omphalocele: Multiple bowel loops (+/- liver) are seen herniating through a membrane-covered midline abdominal defect.
  • 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 edema (present in almost all hydropic infants) > 5 mm
  • Ascites
  • Pleural effusions
  • Pericardial effusions
• Other potential ultrasound findings:
  • Polyhydramnios
  • Increased nuchal translucency
  • Increased placental thickness

Placental abnormalities

Abnormal placentation:

Abnormal placentation refers to abnormal implantation of the placenta. 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 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.

Placental abruption:

Placental abruption refers to the premature separation of the placenta, 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
• Separation of placental edges from the uterus
• Placental thickening

Fluid abnormalities

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

• Polyhydramnios: too much fluid (SDP > 8 cm or AFI > 24 cm)
• Oligohydramnios: too little fluid (SDP < 2 cm or AFI < 5 cm)
• Anhydramnios: no fluid (no measurable pockets of fluid)

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.

Nonobstetric imaging during pregnancy

• Ultrasound and MRI are the preferred modalities due to the lack of radiation exposure.
  • Example: Abdominal ultrasound is the preferred initial test for appendicitis over CT.
• Chest X-ray:
  • Abdomen should be shielded.
  • Only done when absolutely necessary (e.g., clinical deterioration with concern for 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 is considered relatively safe.
  • Risks/benefits should be carefully weighed and discussed with the individual.

Clinical Relevance

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

Diagnosing abnormal pregnancies

• Ectopic pregnancy: pregnancies that have implanted outside the uterine cavity, most commonly in the fallopian tubes. Ectopic pregnancies are nonviable and may rupture as they grow, leading to life-threatening maternal hemorrhage. Individuals typically present with lower abdominal pain and/or abnormal bleeding; a urine pregnancy test will be positive. Management is either with methotrexate or surgical excision, depending on the case.
• Molar pregnancy: arises from abnormal fertilization, resulting in abnormal embryonic genetics. The classic clinical presentation is hyperemesis (severe nausea and vomiting), 1st trimester vaginal bleeding, and enlarged uterus 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.

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 (FGR): a condition, also known as IUGR, referring to poor fetal growth in utero due to environmental factors. 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, 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 edema, ascites, pleural effusions, or pericardial effusions. Hydrops fetalis is often caused by maternal antibodies against fetal RBCs leading to severe fetal anemia, although there are several nonimmune etiologies as well (e.g., infection with 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 begins dilating, the placenta 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 prior to the onset of labor.
• PAS: describes a spectrum in which the placenta is abnormally and firmly adherent to the uterine wall. In this situation, the placenta is unable to detach postpartum and continues to bleed, often heavily, resulting in life-threatening hemorrhage. The 3 degrees of PAS are placenta accreta, increta, and percreta. Diagnosis is via ultrasound, and management is usually surgical, often including a planned hysterectomy at the time of cesarean delivery.

Abnormal amniotic fluid volumes

• 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. Major risks include cord prolapse and placental abruption at the time of membrane rupture.
• Oligohydramnios: a condition characterized by pathologically low amniotic fluid volumes. 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. Management involves close antenatal monitoring because significant complications are possible, including FGR, preterm delivery, and developmental deformities.