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Monoamniotic twins are identical twins that share the same amniotic sac within their mother’s uterus. Monoamniotic twins are always identical, and always monochorionic as well (sharing the same placenta), and are sometimes 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 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.
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.
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.
Regular and aggressive fetal monitoring is recommended for cases of monoamniotic twins to look for cord entanglement beginning after viability. Many women enter inpatient care, with continuous monitoring, preferably in the care of a perinatologist, an obstetrician that specialises in high risk pregnancies.
All monoamniotic twins are delivered prematurely by cesarean section, since the risk of cord entanglement and/or cord compression becomes too great in the third trimester. The cesarean is usually performed at 32, 34 or 36 weeks. Many monoamniotic twins experience life-threatening complications as early as 26 weeks, motivating immediate delivery. However, delivery around 26 weeks is associated with life-threatening complications of preterm birth. Steroids may be administered to stimulate the babies' lung development and decrease the risk of infant respiratory distress syndrome. Natural birth rather than cesarean section causes cord prolapse, with the first baby delivered pulling the placenta shared with the baby being left inside.