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Monochorionic Monoamniotic Twin Gestations
Introduction
In addition to the general risks associated with twin pregnancies and with specific risks of monochorionic gestations, monoamniotic twin gestations face the unique risk of cord entanglement and are at increased risk of fetal demise. Ultrasound (US) plays a vital role in prenatal determination of chorionicity and amnionicity and in the prenatal management of monochorionic monoamniotic twin pregnancies.
Disease
Definition
The term monochorionic refers to a multiple gestation with one placental disk (or chorion), whereas the term monoamniotic describes the presence of a single amniotic cavity. A monochorionic monoamniotic twin pregnancy is one in which there is one shared placenta and a single amniotic sac containing two fetuses. By definition, monochorionic monoamniotic twin pregnancies are monozygotic.
Prevalence and Epidemiology
Although twin gestations accounted for almost 3.4% of live births in the United States in 2014, the estimated incidence of monoamniotic twins is only 1 : 10,000 pregnancies. The frequency of monozygotic twins is constant worldwide at 4 : 1000 births. More than two-thirds of monozygotic twin gestations have monochorionic placentation; however, monoamnionicity affects less than 5% of monozygotic twin gestations. The literature shows that monoamniotic gestations are more common following assisted reproductive technologies such as intracytoplasmic sperm injection.
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