On ultrasound, it is an anechoic (dark) space surrounded by a hyperchoic (white) rim.
It is spherical in shape, and usually located in the upper uterine fundus.
Characteristics on ultrasound
Gestational sacs can be identified via ultrasound and are generally identified by the following 4 characteristics: 1. Round or elliptical shape in longitudinal and transverse views 2. Surrounded by an echogenic rim (choriodecidual reaction) 3. Gestational sac is in uterine fundus 4. Sac is not directly midline, but implanted eccentrically (to one side of the uterine cavity line, without displacing it).