What is the size of the gestational sac? The gestational sac is a bag-like structure formed by the initial development of the embryo. It contains nutrients, fixes the embryo, and protects the embryo from damage. Gestational sac size is often thought to reflect fetal development trends. The measurement process of gestational sac size is usually obtained through ultrasound or B-ultrasound examination. The size of the gestational sac is compared between the results of the two examinations to confirm the growth rate of the gestational sac. If the gestational sac is growing at an inconsistent rate, the expectant mother may need further medical testing. When the size of the gestational sac becomes stable, a visual examination of the embryo can be carried out at approximately 7 weeks to confirm the gestational age.