What is "gestational sac"

n. (context medicine English) The intrauterine structure around an embryo in the very early stage of a fetus that can be used to determine if an intrauterine pregnancy exists until the embryo is identified.

The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis it consists of the extra-embryonic coelom, also called the chorionic cavity. By approximately 9 weeks of gestational age the amniotic sac has expanded to occupy the majority of its volume, eventually filling the entire space. The gestational sac is usually contained with the uterus. It is the only available structure that can be used to determine if an intrauterine pregnancy exists until the embryo is identified.

On obstetric ultrasound, the gestational sac is a dark ("anechoic") space surrounded by a white ("hyperchoic") rim.