During early embryogenesis, it consists of the extraembryonic coelom, also called the chorionic cavity.

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

The gestational sac is spherical in shape, and is usually located in the upper part (fundus) of the uterus.

By approximately nine weeks of gestational age, due to folding of the trilaminar germ disc, the amniotic sac expands and occupy the majority of the volume of the gestational sac, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer between the parietal somatopleuric and visceral splanchnopleuric layer of extraembryonic mesoderm.

The mean sac diameter[3] can effectively estimate the gestational age[4] between 5 and 6 weeks, with an accuracy of about +/- 5 days.