What does the term 'chorionicity' refer to in obstetric ultrasound?

Chorionicity is a term used to describe the number of chorionic sacs present in a pregnancy, particularly in the context of multiple gestations. Each chorionic sac can contain one or more embryos, depending on whether the pregnancy is monozygotic (from a single fertilized egg) or dizygotic (from two separate fertilized eggs).

In multiple pregnancies, understanding chorionicity is crucial as it provides insight into the potential risks and management of the pregnancy. For instance, pregnancies with multiple chorionic sacs are usually considered to have a lower risk of complications compared to those with a single chorionic sac, where twins share the same placenta.

The number of embryos could relate to chorionicity in some cases, but it does not directly define it. The size of the placenta and the health of the fetus are important aspects of obstetric ultrasound as well, but they do not pertain to the concept of chorionicity itself. Therefore, recognizing chorionicity is an essential factor for optimal care and monitoring of pregnancies, especially in multiple gestations.