Twins sharing a single placenta, known as a monochorionic pregnancy, arises from a single fertilized egg that splits after conception. This unique placental arrangement occurs in about 70% of identical twin pregnancies. A shared placenta introduces specific considerations and potential risks for the developing fetuses.
Types of Shared Placenta Pregnancies
Monochorionic pregnancies are primarily categorized by the number of amniotic sacs present. The most common type is monochorionic diamniotic (MCDA), where twins share one placenta but each develops within its own separate amniotic sac. This configuration results from the embryo splitting between days 4 and 8 after fertilization.
A less common and higher-risk type is monochorionic monoamniotic (MCMA), where twins share both a single placenta and a single amniotic sac. This occurs when the embryo splitting happens later, typically after the ninth day following fertilization.
Major Complications of Shared Placenta
The shared placental connection in monochorionic pregnancies can lead to several specific and serious complications. One significant risk is Twin-to-Twin Transfusion Syndrome (TTTS), which affects approximately 10-15% of MCDA twins. This condition occurs when there is an imbalance in blood flow through connecting blood vessels within the shared placenta. One twin, the recipient, receives too much blood, leading to an increased volume of blood, while the other, the donor, receives too little, often resulting in anemia. The recipient twin can develop an enlarged bladder and excessive amniotic fluid, while the donor twin may have a small or absent bladder and very little amniotic fluid.
Another complication is Selective Fetal Growth Restriction (sFGR), where one twin receives an insufficient share of the placenta, causing a notable disparity in growth between the two fetuses. The smaller twin may also experience issues with blood flow due to the disproportionate distribution of placental resources.
Twin Reversed Arterial Perfusion (TRAP) sequence is a rare but severe condition where one twin, the “acardiac” twin, lacks a functioning heart and often has severe malformations. In these cases, the healthy twin’s heart must work to pump blood for both itself and the malformed twin through abnormal vascular connections in the shared placenta. This places an immense strain on the healthy twin’s cardiovascular system, risking heart failure.
Umbilical cord entanglement is a concern primarily for MCMA twins due to their shared amniotic sac. Without a dividing membrane, the umbilical cords of both fetuses can intertwine, posing a risk of compression or knotting that can restrict blood flow. Beyond these unique conditions, monochorionic pregnancies also have a heightened risk of general complications such as preterm birth and an increased likelihood of stillbirth.
Monitoring and Treatment Approaches
Frequent and specialized monitoring is employed to detect and manage the risks associated with monochorionic pregnancies. Regular ultrasound surveillance is a primary part of this care, typically starting around 16 weeks of gestation and often performed every two weeks. These ultrasounds meticulously assess fetal growth, amniotic fluid levels, and blood flow patterns in both twins to identify early signs of complications like TTTS or sFGR. Fetal echocardiograms are also recommended for all monochorionic twins due to an increased risk of congenital heart disease.
When complications arise, various intervention strategies are available. For severe TTTS, fetoscopic laser ablation is often the preferred treatment. This procedure involves using a laser to seal off the abnormal blood vessel connections on the placenta, thereby balancing the blood flow between the twins. Amnioreduction, the removal of excess amniotic fluid from the recipient twin’s sac, can temporarily relieve pressure and discomfort, though it does not address the underlying cause of TTTS.
In cases of severe sFGR or TRAP sequence, or if one twin has a severe anomaly, selective reduction or feticide may be considered. These procedures reduce the number of fetuses in the pregnancy to improve the chances of survival and health for the remaining twin. For risks like umbilical cord entanglement, close monitoring and planned early delivery may be necessary to mitigate potential harm. The management of these complex pregnancies requires specialized care from a maternal-fetal medicine specialist.
Outlook and Post-Birth Care
Despite the inherent risks, many monochorionic twin pregnancies result in positive outcomes with diligent monitoring and timely interventions. Advances in prenatal diagnosis and fetal therapy have contributed to more favorable prognoses. The long-term outlook for babies born from monochorionic pregnancies depends significantly on whether complications occurred during pregnancy and their severity.
Following birth, especially if complications arose, babies from monochorionic pregnancies may require a stay in the neonatal intensive care unit (NICU) for specialized support. This care can address needs related to prematurity, low birth weight, or specific conditions that developed in utero. Continued post-birth follow-up with pediatric specialists may be necessary to monitor their development and address any lingering health concerns.