Risks Associated With MoMo (Monoamniotic Monochorionic) Twins

There are many risks associated with a twin pregnancy, but some of them only affect certain kinds of twins. MoMo twins are monozygotic multiples that develop in a single, shared amniotic sac. This situation causes risk to the babies due to cord entanglement.

The term MoMo is short for Monoamniotic Monochorionic. It describes twins that develop with a single chorion and a single amniotic sac. The amniotic sac is the bag of waters that contains the fetus, while the chorion is the outer membrane.

MoMo twins develop from a single egg/sperm combination which splits into two. When the split is delayed, usually a week or so after conception, the process of growing a placenta, chorion, and amniotic sac has already begun, and the two embryos will develop within a single, shared sac. Only about 1 percent of twin pregnancies will occur in this manner. The majority of monozygotic twins will develop with separate sacs, or sometimes with separate amnions within a shared chorion. (These are described as monochorionic-diamniotic or MoDi.)

Ultrasound is the only way to detect MoMo twins. During a twin pregnancy, most mothers are routinely monitored with ultrasound. Doctors will look for the presence of a dividing membrane to indicate that the twins are in separate sacs. The lack of a membrane or a thin or vague line may prompt further analysis to confirm the situation.

The twin fetuses connect to the placenta via their umbilical cords. Resting together in the same sac puts them at risk for cord entanglement or cord compression. The umbilical cords provide a vital lifeline to the babies, supplying blood and nutrients that help them grown and develop. As the babies move around in the uterus, the cords can cross or press against each other, cutting off the supply. It can be a life-threatening situation. The longer the cords are entwined, the greater the risk of damage to the cords, and the risk of death for one or both babies increases.

Fortunately, modern technology allows doctors to observe babies in the womb and monitor the situation. High-resolution ultrasounds, doppler imaging, and non-stress tests help to assess symptoms and identify potential cord problems. Cord entanglement and compression are usually slow processes, so parents and medical caregivers have time to make decisions. Some situations will require such close monitoring that the expectant mother must remain hospitalized.

There is no approved treatment or procedure to fix the situation. The only resolution is the delivery of the babies. Virtually all MoMo babies are born prematurely. Doctors have to balance the risks of the babies' condition in the womb versus the consequences of prematurity.

If cord compression occurs early in the pregnancy, the babies may not be able to survive. Some doctors elect to schedule delivery of MoMo babies at 32, 34, or 36 weeks, believing that the womb environment is simply too dangerous past that point in time. Sometimes steroids may be administered to boost the babies' lung development and improve their chances of surviving outside the womb.

A cesarean section is mandated for MoMo babies to avoid cord prolapse, a situation that occurs when the second babies cord is expelled as the first baby is delivered.

• MoMo babies are always the same sex: either two boys or two girls. Like virtually all monozygotic twins, they are the same gender because they derive from the same gene set. (No cases of the chromosomal abnormality that generates gender disparity in monozygotic twins has been identified in MoMo twins.)
• MoMo twins are very rare. Only 1 percent of all twin pregnancies will be monoamniotic.
• After 24 weeks, the survival rate of MoMo twins is about 75 to 80 percent.
• A new treatment is being explored. Sulindac is a drug that reduces the amount of amniotic fluid and reducing the space in which the babies can move around.
• Mothers of MoMo multiples should be cared for by a perinatologist (obstetrician specializing in high-risk pregnancies), or should at least consult with a doctor experienced with MoMo twins.
• MoMo twins are often misdiagnosed in the early weeks of pregnancy when the membrane is so thin as to be nearly invisible. Often a later ultrasound reveals a dividing membrane confirming that twins are actually MoDi (Monochorionic, Diamniotic).