Chorionicity of Multiple Gestations

Introduction

In 2014, the twin birth rate in the United States rose to an all-time high of 33.9 per 1000 live births. Multiple gestations result from either the fertilization of multiple ova or the division of a single fertilized ovum into more than one fetus. The terms monozygotic and dizygotic refer to the number of ova leading to a multifetal gestation. A monozygotic pregnancy results from a single fertilized ovum that divides into more than one fetus. In contrast, dizygotic pregnancies originate from the fertilization of two separate ova and are genetically dissimilar.

In contrast to zygosity, which refers to the genetic constitution of a twin pregnancy, the terms chorionicity and amnionicity describe the placentation and membrane composition of a pregnancy. More so than zygosity, the determination of chorionicity and amnionicity is essential in the clinical management of multiple gestations because monochorionic multiples, whether diamniotic or monoamniotic, are at increased risk of adverse outcomes.

Normal Anatomy