The significance of the shape of the sac, also known as the gestational sac, is generally to determine the development of the embryo. The gestational sac is formed when the primordial placental tissue is encapsulated, and can be seen by a woman on a B-mode ultrasound around 35 days into her pregnancy. The shape of the gestational sac is of some significance. By looking at the size, shape and location of the gestational sac on a B-mode ultrasound, it is usually possible to determine whether the pregnancy is intrauterine or ectopic and whether the embryo is viable. The normal shape of the gestational sac is round or oval, which is relatively full. If the shape of the gestational sac is irregular and the sac is collapsed, the embryo may not be developing well. The size of the gestational sac can also be used to determine whether the degree of embryonic development is in accordance with the number of days of pregnancy, based on the duration of pregnancy and the shape of the gestational sac to determine whether the embryo is developing normally.