Twin pregnancies present unique considerations and complexities. While many twin pregnancies progress without significant issues, monoamniotic twins are rare and require specialized attention due to their unique shared environment.
Defining Monoamniotic Twins
Monoamniotic twins, often called MoMo twins, are a unique form of identical twin pregnancy. They develop within a single amniotic sac and share a single placenta, making them monochorionic-monoamniotic. “Monochorionic” indicates the presence of one chorion, which gives rise to a single placenta for both fetuses. “Monoamniotic” signifies that both twins reside within the same fluid-filled sac, without a separating membrane.
How Monoamniotic Twins Form
The formation of monoamniotic twins stems from a specific timing in the early development of a fertilized egg. All identical twins originate from a single zygote that splits into two embryos. In the case of MoMo twins, this division occurs relatively late, specifically between day 8 and day 13 after fertilization. If the split happens within this narrow window, the developing embryos will share both the chorion, which forms the placenta, and the amnion, which forms the inner sac containing the amniotic fluid. This late separation prevents the formation of two distinct amniotic sacs.
Distinguishing Monoamniotic Twins from Other Types
Understanding monoamniotic twins requires differentiating them from other common twin classifications. Dichorionic-diamniotic (DiDi) twins are the most common type, possessing separate placentas and separate amniotic sacs. These can be either fraternal or identical. Another type is monochorionic-diamniotic (MoDi) twins, which are always identical. These twins share a single placenta but develop within their own separate amniotic sacs, divided by a thin membrane. Monoamniotic twins stand apart as they are the only type where identical twins share both a single placenta and a single amniotic sac, lacking any separating membrane between them.
Unique Pregnancy Risks
The shared environment of monoamniotic twins gives rise to distinct and serious pregnancy risks. A primary concern is cord entanglement, where the umbilical cords of the twins can become intertwined due to the absence of a dividing membrane. This entanglement can restrict blood flow to one or both fetuses, potentially leading to oxygen deprivation or even fetal demise.
Another significant risk is twin-to-twin transfusion syndrome (TTTS), which occurs in monochorionic pregnancies where there are shared placental blood vessels. In TTTS, blood flow becomes unevenly distributed between the twins, with one twin receiving too much blood (recipient) and the other too little (donor). This can lead to severe health issues for both, including heart problems in the recipient and growth restriction in the donor.
Monoamniotic twin pregnancies also carry a higher likelihood of selective fetal growth restriction, where one twin grows significantly slower than the other. Furthermore, preterm birth is a common complication in MoMo pregnancies, often occurring before 37 weeks of gestation. This early delivery can lead to various health challenges for the newborns, including respiratory distress and developmental issues.
Managing a Monoamniotic Twin Pregnancy
Managing a monoamniotic twin pregnancy involves a highly specialized and intensive medical approach due to the elevated risks. Frequent and rigorous monitoring is a cornerstone of care, typically involving regular ultrasounds, sometimes daily or several times a week, especially in the later stages of pregnancy. These scans are performed to closely observe fetal growth, amniotic fluid levels, and, most importantly, to detect signs of cord entanglement or compression.
Given the substantial risks, hospitalization is a common recommendation for expectant mothers carrying monoamniotic twins, particularly beginning in the third trimester. This inpatient monitoring allows for immediate medical response if complications arise, offering continuous surveillance of fetal well-being. The close proximity to medical staff and equipment is an important aspect of ensuring the best possible outcomes.
Delivery planning for monoamniotic twins is carefully orchestrated to mitigate risks. A cesarean section is almost always the preferred method of delivery to avoid the stress of labor, which could worsen cord compression or entanglement. The optimal timing for delivery is typically between 32 and 34 weeks of gestation. This early delivery aims to balance the risks of prematurity against the increasing danger of cord complications as the twins grow larger and their movements become more pronounced.