Gestational sac

The gestational sac (or gestation sac) is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy (IUP) exists, until the embryo is identified.

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.

The mean gestational sac diameter (MGD) is an effective estimate of gestational age between 5 and 6 weeks, with an accuracy of about +/- 5 days.

The yolk sac and embryo should be readily identified when the gestational sac reaches a certain size and the yolk sac is 20 mm in size or the fetal pole is 25 mm in size.

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).