An Overview of Monoamniotic-Monochorionic (MoMo) Twins

The terms MoMo and mono-mono are short for monoamniotic-monochorionic and refer to twins that are monozygotic multiples—those who develop in a single amniotic sac and share a placenta. The words literally translate to mean a single chorio (outer membrane surrounding an embryo) and a single amniotic sac (the bag of waters that contains the fetuses). This situation is very rare and may cause risk to the babies due to cord entanglement and other issues.

Identical (monozygotic) twins develop from a single egg-sperm combination that splits into two. If 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, resulting in MoMo twins.

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, which are described as monochorionic-diamniotic or MoDi.

MoMo babies always have identical features and are of the same sex because they derive from the same gene set. No cases of the chromosomal abnormality that sometimes results in boy-girl sets of monozygotic twins have ever been identified in MoMo twins.

During a twin pregnancy, most mothers are routinely monitored with ultrasound. Doctors will look for the presence of a dividing membrane that indicates 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. Ultrasound is the only way to detect 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 monochorionic-diamniotic.

Monoamniotic-monochorionic twins face many potential health risks throughout pregnancy. As such, 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.

The following health risks are what makes these pregnancies concerning:

The twin fetuses connect to the shared placenta via their own umbilical cords, which supply blood and nutrients that help them grown and develop. As the babies move around in the same amniotic sac in the uterus, the cords can cross or press against each other, cutting off these vital lifelines.

This can be a life-threatening situation. The longer the cords are entwined or compressed, the greater the risk of damage to the cords—and the greater the risk of death for one or both babies.

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, leaving the donor twin undernourished (smaller and often anemic). Weight inequality can be a concern in MoMo twins as it can lead to growth restrictions in the womb, known as intrauterine growth restriction (IUGR).

A doctor can diagnose TTTS in any set of twins by examining fluid levels in their amniotic sacs. However, the fact that MoMo twins only have one sac makes a TTTS diagnosis much more difficult. Comparing the physical development of both of the twins is the only way to diagnose this condition prior to birth.

MoMo twins can be affected by amniotic fluid levels that are either too low (oligohydramnios) or too high (polyhydramnios).

Low blood supply in one of the twins will lead to not enough amniotic fluid, which limits movement, bladder size, and overall fetal growth, in addition to decreasing the protection from compression of the umbilical cord in the uterus.

A larger than normal blood supply will result in excess amniotic fluid, leading to an enlarged bladder and the possibility of heart failure. 

Unfortunately, low birth weight is independently linked to reduced odds of survival and a higher risk for disabilities and health problems in life. MoMo twins have four times the risk of low birth weight as compared to pregnancies in which each fetus has a placenta of its own.

After 24 weeks, the survival rate of MoMo twins is about 75 percent to 80 percent.

Many MoMo twins experience life-threatening complications as early as 26 weeks resulting in spontaneous preterm delivery or an earlier than planned Cesarean section (C-section). 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 close monitoring that the expectant mother must remain hospitalized.

Doctors have to balance the risks of the babies' condition in the womb versus the consequences of prematurity.

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

If cord compression occurs early in the pregnancy, the babies may not be able to survive. The risk of cord entanglement and compression is simply too great after 34 weeks. For this reason, all MoMo twins are delivered by C-section at around 34 weeks, making them preemies. Sometimes steroids may be administered to boost the babies' lung development and improve their chances of surviving outside the womb.

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.