Monoamniotic twins occur when a single fertilized ovum (egg) results in identical twins that share a common placenta and amniotic sac.
Monoamniotic twins are very uncommon, representing approximately 1% of identical twins and less than 0.1% of all pregnancies.
Due to a shared amniotic space, monoamniotic twins commonly experience entanglement of their umbilical cords. In some instances, this entanglement 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. Therefore, these twins are closely monitored throughout pregnancy, usually involving inpatient hospitalization from a very premature time. Given an association between monoamniotic twins and physical malformations, these twins undergo fetal echocardiography in addition to usual ultrasound-based assessments.
With contemporary management, survival rates for monoamniotic twins are around 90%. These twins are necessarily born at a premature gestational age, even in the absence of identified complications before birth. In addition to physical malformations that may be present, prematurity is a major factor in determining long-term health.