A diagnosis of a bilobed placenta often prompts questions about potential risks to the mother and the developing fetus. The placenta serves as the temporary life support system, facilitating the exchange of nutrients, oxygen, and waste products. While this structural variation may sound alarming, the majority of pregnancies involving a bilobed placenta result in healthy, normal outcomes. The diagnosis signals the need for closer prenatal attention and specialized management to monitor for associated complications.
What is a Bilobed Placenta?
A bilobed placenta, also known as a bipartite or duplex placenta, is a variation where the placental tissue is divided into two distinct discs or lobes. These two lobes are usually of roughly equal size, separated but connected by a thin bridge of fetal membranes and blood vessels. This morphology differs from the typical, singular, and uniformly discoid shape expected in an uncomplicated pregnancy.
The estimated incidence of this placental variation is relatively low, occurring in up to 4% of all pregnancies. The formation is thought to be related to the implantation process, possibly resulting from localized atrophy or poor vascularization in a section of the uterus early in development. The umbilical cord may insert into one of the lobes, or more commonly, into the connecting membrane between the two lobes.
Potential Risks and Associated Complications
Retained Placenta and Hemorrhage
The primary concerns with a bilobed placenta stem from its altered structure, which increases the probability of certain complications during and immediately following delivery. The two-lobed structure increases the risk of retained placental tissue after the delivery of the baby. A retained fragment prevents the uterus from contracting effectively after birth, which is a leading cause of postpartum hemorrhage (PPH) involving excessive maternal blood loss. Physicians must carefully inspect the delivered placenta to ensure both lobes and all connecting membranes have been successfully expelled.
The bilobed structure is also associated with an increased incidence of bleeding earlier in the pregnancy, as well as conditions like polyhydramnios or placental abruption.
Anomalous Cord Insertion and Vasa Previa
A significant set of risks involves the umbilical cord insertion point. A bilobed placenta is associated with an increased chance of anomalous cord insertion, particularly velamentous cord insertion. This occurs when the umbilical cord vessels separate and travel through the fetal membranes before reaching the placental tissue, leaving them unprotected by the usual Wharton’s jelly.
The unprotected vessels are concerning if they cross over the internal cervical opening, a condition known as vasa previa. In vasa previa, these fetal vessels are positioned between the baby and the birth canal. If the membranes rupture during labor, these exposed vessels can tear, leading to rapid and catastrophic blood loss for the fetus. The increased risk of vasa previa is the most serious complication associated with a bilobed placenta.
Fetal Well-being
While a bilobed placenta itself is not typically linked to an increased risk of congenital fetal anomalies, secondary complications can impact fetal well-being. For example, velamentous cord insertion may be associated with fetal growth restriction or preterm delivery. The overall increased risk of these complications necessitates heightened surveillance throughout the remainder of the pregnancy.
Clinical Monitoring and Delivery Strategies
The management of a bilobed placenta begins with early and accurate prenatal diagnosis, typically achieved through routine ultrasound examinations. Once the variation is identified, a closer surveillance strategy is adopted to specifically look for the associated risks, especially cord insertion anomalies. Targeted ultrasounds, often utilizing color Doppler technology, are used to carefully track the course of the umbilical cord vessels, particularly in the third trimester.
This detailed imaging is crucial to rule out or confirm the presence of vasa previa, which dramatically alters the delivery plan. If vasa previa is confirmed, the standard of care involves scheduling a planned Cesarean section before the onset of labor and before the membranes spontaneously rupture. This proactive approach prevents the potentially fatal compression or rupture of the fetal vessels.
If vasa previa is not present, a vaginal delivery may still be considered, but the delivery team remains on high alert for complications during the third stage of labor. Following the birth of the baby, the primary focus shifts to the immediate delivery of the entire placenta. The physician must meticulously examine the expelled placenta to confirm that all tissue, including both lobes and the connecting membrane, has separated cleanly from the uterine wall. If there is any suspicion that a fragment has been retained, manual exploration of the uterus may be necessary to remove the remaining tissue and prevent postpartum hemorrhage.