This article has been moved over to Wikipedia and will soon be deleted if the Lord is merciful.
You can see the article here: Wikipedia:Gestational sac
The gestational 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 space surrounded by a hyperchoic 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 20mm in size or the fetal pole is 25mm in size.