Monoamniotic twins

Monoamniotic twins are identical twins that share the same amniotic sac within their mother’s uterus. Monoamniotic twins are always identical, always monochorionic and are usually termed Monoamniotic-Monochorionic ("MoMo") twins. They also share the placenta, but have two separate umbilical cords. Monoamniotic twins develop when an embryo does not split until after formation of the amniotic sac, at about 9-13 days after fertilization. Monoamniotic triplets or other monoamniotic multiples are possible, but extremely rare. Other obscure possibilities include multiples sets where monoamniotic twins are part of a larger gestation such as triplets, quadruplets, or more.

Monoamniotic twins are rare, with an occurrence of 1 in 35,000 to 1 in 60,000 pregnancies.

The survival rate for monoamniotic twins has been shown to be as high as 81% to 95% in 2009 with aggressive fetal monitoring, although previously reported as being between 50% to 60%. Causes of mortality and morbidity include:

Ultrasound is the only way to detect MoMo twins before birth. It can show the lack of a membrane between the twins after a couple of weeks' gestation, when the membrane would be visible if present.

Further ultrasounds with high resolution doppler imaging and non-stress tests help to assess the situation and identify potential cord problems.

There is a correlation between having a single yolk sac and having a single amniotic sac. However, it is difficult to detect the number of yolk sacs, because the yolk sac disappears during embryogenesis.

Cord entanglement and compression generally progress slowly, allowing parents and medical caregivers to make decisions carefully.

Only a few treatments can give any improvements.

Sulindac has been used experimentally in some monoamniotic twins, lowering the amount of amniotic fluid and thereby inhibiting fetal movement. This is believed to lower the risk of cord entanglement and compression. However, the potential side effects of the drug have been insufficiently investigated.

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