Monochorionic Monoamniotic (MoMo) twins are the rarest and highest-risk form of identical twin pregnancy. This classification means the twins share both a single placenta and a single amniotic sac, the fluid-filled membrane protecting the developing fetuses. Occurring in only about one in 35,000 to 60,000 pregnancies overall, MoMo twins require specialized medical attention. The shared sac introduces significant complications that necessitate rigorous prenatal monitoring and careful management.
The Biological Definition of MoMo Twins
Timing of the Split
MoMo twins are exclusively identical (monozygotic), originating from a single fertilized egg that split into two embryos. The type of twinning is determined by the precise timing of the split after fertilization. If the zygote splits very early (within the first three days), the twins develop entirely separate structures (Dichorionic Diamniotic or DiDi).
Monochorionic Monoamniotic Structure
The MoMo classification occurs because the split happens relatively late, generally between day eight and day thirteen post-fertilization. By this time, the outer membrane (chorion) has already formed, causing the twins to share one chorion and thus one placenta (monochorionic). Crucially, the split also happens after the inner membrane (amnion) has formed, resulting in the twins sharing a single amniotic sac. This structure is distinct from Monochorionic Diamniotic (MoDi) twins, which have a shared placenta but two separate amniotic sacs.
Critical Risks Associated with Sharing an Amniotic Sac
The primary danger in a MoMo pregnancy arises directly from the shared, undivided amniotic sac. The two fetuses are free to move without any barrier, allowing their umbilical cords to cross, twist, and knot together. This cord entanglement occurs to some degree in almost every monoamniotic pregnancy. Entanglement can lead to cord compression, restricting or cutting off the blood and nutrient supply. Because the cords are the sole lifeline to the shared placenta, compression can cause fetal distress, growth restriction, and, in severe cases, the death of one or both twins.
MoMo twins are also at risk for complications common to all monochorionic pregnancies, such as Twin-to-Twin Transfusion Syndrome (TTTS) and selective fetal growth restriction (sFGR). TTTS involves an imbalance of blood flow through the shared placenta, causing one twin to receive too much blood and the other too little.
Specialized Prenatal Monitoring and Management
Due to the continuous and unpredictable threat of cord complications, MoMo pregnancies require rigorous prenatal surveillance. Early diagnosis is confirmed by first-trimester ultrasound when the absence of a dividing membrane is noted. The standard of care involves frequent ultrasound examinations, often weekly starting in the second trimester, to monitor fetal growth, amniotic fluid levels, blood flow, and signs of cord issues.
Inpatient hospitalization is often recommended between 24 and 28 weeks of gestation. The purpose of this prolonged hospital stay is to provide continuous fetal monitoring that cannot be safely achieved at home. This involves performing daily or twice-daily Non-Stress Tests (NSTs) and biophysical profiles. NSTs track the twins’ heart rates in response to movement, offering immediate indication of distress, such as a sudden drop signaling cord compression. Steroids, such as betamethasone, are administered during the inpatient stay to accelerate lung development, preparing them for the high likelihood of a premature delivery.
Delivery Timing and Neonatal Care
The delivery of MoMo twins is carefully timed to balance the inherent risks of the shared intrauterine environment against the risks associated with prematurity. The optimal window for delivery is generally between 32 and 34 weeks of gestation. Delivery is usually scheduled within this window because the risk of a catastrophic event, particularly severe cord entanglement, increases significantly past the 32-week mark.
The preferred method of delivery for MoMo twins is almost always a Cesarean section. This choice is made to prevent the second twin from experiencing umbilical cord compression or entanglement complications that could occur during a vaginal birth.
Given the planned early delivery, the twins are born moderately premature, meaning immediate and specialized neonatal care is required. Almost all MoMo twins require admission to the Neonatal Intensive Care Unit (NICU). Common immediate post-birth issues include respiratory distress syndrome due to underdeveloped lungs, poor temperature regulation, and feeding difficulties. The coordinated care between the obstetric team and the neonatology team is essential to manage the challenges of prematurity and ensure the best long-term outcomes for both infants.