The placenta is an important structure for fetal growth and development during embryonic and fetal life. Abnormalities in its structure, function, or implantation can result in serious and potentially fatal complications for both the fetus and mother.
Placental structure and circulation
- Chorionic villi provide a large surface area for maternal-fetal exchange.
- Spiral arteries (maternal) fill the intervillous spaces in the decidua basalis layer of the endometrium:
- Bring in oxygenated blood for the fetus
- The spiral arteries “rupture” and become large spaces called lacunae, which:
- Are extremely low-resistance areas
- Do not have the ability to regulate blood flow through the organ
- 2 umbilical arteries bring deoxygenated blood from the fetus to the placental chorionic villi.
- Exchange of gases and molecules occurs between the fetal blood in the chorionic villi and maternal blood in the lacunae, across the placental barrier.
- Umbilical vein transports oxygenated blood back to the fetus.
- Maternal veins carry deoxygenated blood back to the maternal circulation.
- Maternal and fetal blood never come into direct contact.
- Fetal hemoglobin has ↑ affinity for O2 compared with maternal hemoglobin → causes O2 to move from maternal RBCs to fetal RBCs
Functions of the placenta
- Gas exchange (O2 and CO2)
- Nutrient exchange
- Fetal waste removal
- Hormone production (a fetal and maternal endocrine organ during pregnancy):
- Human placental lactogen (hPL)
- Chorionic thyrotropin
- Chorionic corticotropin-releasing hormone (CRH)
- Metabolic functions to support the fetus:
- Glycogen and cholesterol synthesis
- Protein metabolism
- Assists in rejection of the maternal immune system
Normal placental implantation
- Early fetal trophoblastic cells invade into the maternal decidua basalis layer of the endometrium:
- Trophoblastic cells should not invade into the myometrium beneath the endometrium.
- When trophoblastic cells do invade into the myometrium, the condition is called placenta accreta.
- Placentas typically implant in the fundal region of the uterus:
- The placental edge should be away from the internal cervical os.
- When the placenta covers the internal cervical os, the condition is called placenta previa.
- When the placental edge is within 2 cm of the cervical os, it is called low-lying placenta.
- The placenta should remain connected to the maternal endometrium, providing O2 and nutrients, until after the delivery of the infant.
- When the placenta separates early, the condition is called placental abruption.
- When the placenta fails to provide adequate O2 or nutrients to the fetus, the condition is called placental insufficiency.
Structural Abnormalities of the Placenta and Umbilical Cord
Bilobate and succenturiate placentas
- Bilobate placenta:
- Separation of the placental forms into nearly equal-sized lobes with the umbilical cord inserted between them
- Incidence: 1 in 350 deliveries
- Multilobate placenta: Placenta has ≥ 3 lobes.
- Succenturiate lobe:
- 1 or more smaller lobes develop.
- Lobes often develop at a distance from the main placenta, but with a vascular connection.
- More common with twins
- Diagnosis: Placentas are easily seen on ultrasound, and abnormal structures are typically identified on routine antenatal ultrasound.
- ↑ Risk of postpartum hemorrhage due to retained placental tissue (e.g., the main lobe delivered but the succenturiate lobe did not)
- Vessels connecting the main placenta with the succenturiate placenta may rupture during labor causing fetal hemorrhage/demise.
- ↑ Risk of vasa previa (see below)
Velamentous cord insertion
- The last few centimeters of the umbilical cord entering the placenta lack the protective Wharton’s jelly.
- The vessels are “exposed” and covered only by the fetal membranes.
- Epidemiology: 1% of all pregnancies
- Risk factors:
- Multiple pregnancies, mainly twin gestations
- Placenta previa
- Diagnosis: Antenatal ultrasound shows a cord that does not insert centrally into the placenta.
- Intrapartum: Continuous fetal heart rate monitoring and C-section delivery should be considered.
- Antepartum: If velamentous insertion overlies the cervix, an elective C-section should be scheduled for 35 weeks gestation.
- Vessels are much more vulnerable to rupture.
- Vascular compression → fetal morbidity and mortality
- Much higher risk of vasa previa and antenatal hemorrhage
- Definition: occurs when fetal blood vessels traverse or run near the internal cervical os
- Epidemiology: 1 in 5200 pregnancies
- Associated risk factors:
- Velamentous umbilical cord
- Succenturiate placental lobes
- Pregnancies conceived via in vitro fertilization (IVF)
- Multiple gestations
- Clinical presentation: triad consisting of:
- Membrane rupture
- Painless vaginal bleeding (fetal blood)
- Fetal bradycardia (HR < 110/min)
- Confirmed by examination of placenta after delivery
- Suspected in antenatal sonogram with color flow doppler
- Pelvic rest (no digital exams, no intercourse; “nothing in the vagina”) antenatally
- Immediate cesarean delivery for clinical bleeding and/or signs of labor
- Complications: blood vessels rupture → bleeding from fetoplacental circulation → fetal exsanguination and death
Other anomalies of the placenta and umbilical cord
- Single umbilical artery (may ↑ the risk of aneuploidy or other associated anomalies)
- Marginal cord insertion:
- Insertion of the cord near the edge of the placenta rather than in the middle
- May ↑ the risk of the cord being torn off during delivery of the placenta
- Umbilical cord loops and/or knots, which may impede blood flow.
- Funic presentation:
- When a loop of the normal umbilical cord is tucked below the fetal presenting part
- ↑ Risk for cord prolapse or fetal distress due to cord compression
- Typically requires C-section
Placenta Accreta Spectrum
Placenta accreta spectrum (PAS) describes a spectrum of abnormal placentation in which the placenta is abnormally and firmly adherent to the uterine wall. The 3 degrees of PAS are:
- Placenta accreta (approximately 65%): The placenta attaches directly to the myometrium due to 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.
Epidemiology and pathogenesis
- The prevalence has been increasing steadily over the past few decades and is now approximately 0.2% of pregnancies.
- Likely due to defective decidualization in an area of scar caused by prior uterine surgery
- Placenta previa after a prior C-section:
- Most important risk factor
- > 60% chance of PAS after 4 C-sections if placenta previa is present
- Prior history of uterine surgery/procedures:
- Cesarean delivery
- Dilation and curettage
- Endometrial ablation
- Infertility procedures
- Maternal age > 35 years
- Postpartum endometritis
- If the placenta requires manual removal
- Typically asymptomatic
- Usually diagnosed on routine antenatal ultrasound
- If missed on ultrasound, PAS is typically diagnosed at delivery when the placenta fails to deliver.
- Massive postpartum hemorrhage can result when manual separation of the placenta is attempted.
- Therefore, when attempting manual removal of the placenta:
- Try to find the natural tissue plane between the placenta and the uterus, and pull it out intact.
- Do not create a plane by ripping the placenta.
- Planned cesarean delivery
- For placenta increta and percreta, concurrent hysterectomy is often required → patients require extensive counseling
- Severe postpartum hemorrhage secondary to retained placental tissue (blood is entering maternal lacunae that are now “open” without the fetal chorionic plate being present)
- Complications related to hypovolemic/hemorrhagic shock, including postpartum hypopituitarism (i.e., Sheehan syndrome)
Placenta previa refers to 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 fetal hemorrhage.
- Previous placenta previa
- Previous cesarean delivery
- Prior uterine procedures (e.g., dilation and curettage)
- Multiple gestations
- Maternal cocaine use
- Typically diagnosed on routine antenatal ultrasound, which shows the placenta over the cervical os
- If missed on ultrasound, may present with painless bright-red bleeding:
- Often occurs after a digital exam or intercourse
- Can be differentiated from placental abruption, which presents with painful bleeding and uterine irritability
- Note: Approximately 90% of previas identified at < 20 weeks gestation will resolve as the lower uterine segment grows, “moving” the placenta further away from the cervical os.
- Resolution is more likely when only the edge of the placenta is touching the edge of the cervix.
- Less likely to resolve when the os is completely covered
- Pelvic rest
- Avoid digital exams (cervical dilation should be assessed visually with a speculum exam).
- Follow with serial ultrasounds to look for resolution.
- Planned cesarean delivery if placenta previa remains at full term (or at the onset of labor)
- Preterm delivery (approximately 45% of patients)
- Antenatal hemorrhage (approximately 50% of patients)
- Postpartum hemorrhage (approximately 25% of patients)
- Risk of recurrence in subsequent pregnancies (4%‒8% of patients)
Uteroplacental insufficiency may be acute or chronic and refers to the inability of the placenta to deliver a sufficient supply of O2 and nutrients to the fetus.
Chronic uteroplacental insufficiency
Chronic uteroplacental insufficiency results in fetal growth restriction and associated complications.
- Etiology/risk factors:
- Maternal vascular disease, especially:
- Chronic hypertension
- Pregestational diabetes
- Severe maternal anemia
- Maternal smoking or cocaine use
- Uterine malformations
- Maternal vascular disease, especially:
- Diagnosis via ultrasound, demonstrating:
- Fetal growth restriction
- Oligohydramnios (frequent)
- Increased vascular resistance within the placenta on Doppler studies
- Patients should be monitored antenatally with frequent nonstress tests and ultrasound to look for signs of fetal decompensation.
- Delivery is indicated when the fetus begins showing signs of distress (e.g., non-reassuring fetal status on testing).
- Typically, if patients are otherwise candidates for vaginal delivery, induction of labor is recommended over planned cesarean delivery, with close intrapartum monitoring.
- If preterm, give a course of steroids (typically IM betamethasone) to help promote fetal lung maturity prior to delivery.
Acute uteroplacental insufficiency
- Periods of ischemia during labor contractions, preventing the delivery of O2 and nutrients at levels required to satisfy fetal requirements
- Note: Relative ischemia occurs during normal labor, but can be magnified with placental pathology.
- Partial or complete placental abruption
- Diagnosis is via abnormalities noted on fetal cardiac monitoring:
- Recurrent late decelerations
- Persistent fetal bradycardia
- Uterus is frequently hypertonic and/or tachysystolic.
- Intrauterine resuscitation techniques:
- Reposition the mother (may relieve vessel compression).
- Give mother IV fluids and O2 therapy to support her circulating O2 capacity.
- ↓ Uterotonics (e.g., pitocin) and/or administer uterine tocolytics (e.g., terbutaline)
- Expedited delivery is indicated if infants fail to respond to resuscitation, via:
- Cesarean delivery (if remote from delivery)
- Operative vaginal delivery (e.g., forceps or vacuum extraction) if candidates are in the 2nd stage of labor
The most common clinical complications of abnormal placentation are antepartum and postpartum hemorrhage, which can be both severe and life threatening, depending on the clinical situation. Additionally, the placenta may become infected or develop a malignancy.
- Antepartum hemorrhage: refers to uterine bleeding that occurs prior to the onset of labor. The most clinically important types of antepartum hemorrhage that should be immediately identified and treated include bleeding from placental abruption (typically painful), a placenta or vasa previa (typically painless), or uterine rupture (in patients with a history of major uterine surgery including a C-section delivery). Bleeding frequently affects both the mother and infant; thus, immediate delivery is indicated in most cases.
- Postpartum hemorrhage: refers to uterine bleeding after the infant is delivered. The 4 primary causes can be remembered as the 4 T’s: Tone (uterine atony, by far the most common), Tissue (retained placental tissue), Trauma (lacerations of the perineum, vagina, or cervix), and Thrombin (coagulation disorders, especially disseminated intravascular coagulation). The diagnosis is typically clinical. The retained placental tissue should be removed manually or surgically.
- Chorioamnionitis (intraamniotic infection (IAI)): a common obstetric complication involving infection and inflammation of the fetal membranes, amniotic fluid, placenta, or the fetus itself. Chorioamnionitis is typically caused by an ascending polymicrobial infection from the lower genitourinary tract, most commonly after the prolonged rupture of membranes. There are specific clinical criteria for diagnosis, including maternal fever. An IAI is managed with antibiotics, and ultimately, delivery, which typically results in its resolution.
- Gestational trophoblastic disease (GTD): a group of pregnancy-related tumors resulting from abnormal placental trophoblastic growth that range from benign molar pregnancies (complete and partial) to neoplastic conditions (e.g., invasive mole, choriocarcinoma, and placental site trophoblastic tumor). Patients will typically have grossly elevated serum β-hCG levels and characteristic ultrasound findings. Treatment involves surgery and/or chemotherapy, usually with methotrexate.
- Cunningham, F.G., Leveno, K.J., et al. (2010). Williams Obstetrics (23rd ed. pp. 36‒72, 557‒585).
- Lockwood, C.J., Russo-Stieglitz, K. (2021). Placenta previa: Epidemiology, clinical features, diagnosis, morbidity, and mortality. In Barss, V.A. (Ed.), UpToDate. Retrieved June 22, 2021, from https://www.uptodate.com/contents/placenta-previa-epidemiology-clinical-features-diagnosis-morbidity-and-mortality
- Silver, R.M. (2021). Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences. In Barss, V.A. (Ed.), UpToDate. Retrieved June 22, 2021, from https://www.uptodate.com/contents/placenta-accreta-spectrum-clinical-features-diagnosis-and-potential-consequences
- Mari, G. (2021). Fetal growth restriction: Evaluation and management. In Barss, V.A. (Ed.), UpToDate. Retrieved June 22, 2021, from https://www.uptodate.com/contents/fetal-growth-restriction-evaluation-and-management