Monoamniotic twins, also known as "MoMo" twins, are identical twins that share the same amniotic sac within their mothers uterus. They are always monochorionic, meaning they share the placenta, but have two separate umbilical cords. Monoamniotic twins develop when an embryo does not split until after the formation of the amniotic sac, which occurs about 9-13 days after fertilization. Monoamniotic twins are very rare, representing approximately 1% of twin pregnancies.

Monoamniotic twins face increased risks compared to other twin pregnancies, with the potential for severe complications such as umbilical cord entanglement or compression that can threaten the survival of one or both babies. Due to the shared amniotic space, monoamniotic twins commonly experience entanglement of their umbilical cords, which can be severe enough to compromise blood supply to the twins, resulting in the loss of one or both of the twins. These instances can be difficult to predict, so monoamniotic twins are closely monitored throughout pregnancy, usually involving inpatient hospitalization from a very premature time.

Other potential complications of monoamniotic twins include reduced blood supply for each baby because of sharing, uneven blood flow that stunts growth in the womb, often affecting only one baby, and twin-to-twin transfusion syndrome (TTTS), the rapid transfer of blood from one baby to the other that can lead to heart failure and death of one or both twins.

It is important to know about monoamniotic twins early on in pregnancy so that the mother and babies can be closely monitored and managed by a skilled, experienced team of medical professionals.