Monoamniotic-Monochorionic Twins (MoMo Twins)

Definition - What does Monoamniotic-Monochorionic Twins (MoMo Twins) mean?

Monoamniotic-monochorionic twins are identical twins that share both an amniotic sac and a placenta. They are rare, occurring in 0.3% of twin pregnancies, and are complicated pregnancies.

FertilitySmarts explains Monoamniotic-Monochorionic Twins (MoMo Twins)

Several possible arrangements of placentas and amniotic sacs exist for identical twins. Monoamniotic means that both fetuses are encased in one amniotic sac and share amniotic fluid, while monochorionic means that both fetuses share one placenta. Each fetus has its own umbilical cord. Because of this unusual arrangement, there are several complications that can arise in mono-mono twins.

As the fetuses are close to each other, and because there is no amniotic membrane separating them, they can easily become tangled in each other’s umbilical cords. This condition is called cord entanglement. Also due to the lack of amniotic membrane, one twin can apply pressure to the other twin’s umbilical cord in a condition called cord compression. This can block the flow of blood, and therefore nutrients and oxygen, and can result in the death of one of the twins. Because mono-mono twins share a placenta, their circulatory systems are also connected in random tangles. This can lead to twin-to-twin transfusion syndrome (TTTS), when one twin gets most of the nutrients, leaving the other twin without. A connected blood supply can also lead to a difference in birth weights and another very serious complication called twin reversed arterial perfusion syndrome (TRAPS). In this syndrome, one twin doesn’t grow a heart and is reliant on the other twin’s heart to pump for it. This puts far more pressure on the twin with the heart, as it’s pumping for both fetuses.

Mono-mono twins can be detected a few weeks into pregnancy, and women pregnant with mono-mono twins will undergo continuous, rigorous monitoring. Monoamniotic twins are delivered early, around 32 weeks because of the risks of cord entanglement and compression. Also because of the shared placenta, vaginal delivery of one twin compromises the other as the first pulls on the placenta. As a result, Caesarean section is usually recommended.

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