Momo (Monoamniotic Monochorionic) Twins Risks

Twin girls in identical dresses sitting on a cough
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The terms MoMo or Mono Mono are short for Monoamniotic Monochorionic and refer to twins that are monozygotic multiples which develop in a single, shared amniotic sac. The words literally translate to mean 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 surrounding an embryo. This situation is very rare and may cause risk to the babies due to cord entanglement.

Why Do Mono Mono Twins Form?

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 processes of growing a placenta, chorion, and amniotic sac are already underway. The two embryos will then develop within a single, shared sac. Only about one 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, which are described as monochorionic-diamniotic or MoDi.

MoMo babies are always the same sex—either two boys or two girls—and always identical. 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 have been identified in MoMo twins.

How the Condition Is Diagnosed

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.

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.

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).


MoMo twins face many potential health risks throughout pregnancy. The following health risks are why it is important to ensure that you receive specialized care by a professional trained in possible MoMo twin risks:

Umbilical Cord Complications

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.

Twin-to-Twin Transfusion Syndrome

MoMo twins are susceptible to twin-to-twin transfusion syndrome (TTTS), which happens when one twin (the donor) essentially provides a blood transfusion to another twin (the recipient). The recipient twin often receives the majority of the nourishment in the womb while the donor twin is left undernourished (smaller and often anemic).

A doctor would diagnose TTTS by examining fluid levels in amniotic sacs and the one amniotic sac of MoMo twins makes a TTTS diagnosis that much more difficult. Comparing the physical development of both of the twins is the only way to diagnose this condition prior to birth.

Preterm Birth

After 24 weeks, the survival rate of MoMo twins is about 75 to 80 percent, however, the risk of cord entanglement and compression is simply too great after 34 weeks. For this reason, all MoMo twins are born prematurely and delivered by cesarean section at around 34 weeks. Unfortunately, many MoMo twins experience life-threatening complications as early as 26 weeks and preterm delivery is often associated with a number of other life-threatening conditions.


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. This is why 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.

As for pharmaceutical treatment, Sulindac is a drug that reduces the amount of amniotic fluid to reduce the space in which the babies can move around. More research is required for this type of treatment option as the possible risks may outweigh the benefits.

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Article Sources
  • Prefume F, Fichera A, Pagani G, et al. The natural history of monoamniotic twin pregnancies: a case series and systematic review of the literature. Prenat Diagn. 2015; 35(3):274-80.
  • Roque H, Gillen-Goldstein J, Funai E, Young BK, Lockwood CJ. Perinatal outcomes in monoamniotic gestations. J Matern Fetal Neonatal Med. 2003;13(6):414-21.