Gestational sac
（重定向自Chorionic cavity）
![](/Images/godic/202501/04/Ultrasound_of_embryo_at_5_weeks3520.png")
![](/Images/godic/202501/04/Ultrasound_of_embryo_at_5_weeks,_colored3520.png")
![Mean gestational sac diameter by gestational age. The blue line is the mean, and the green area delimits the 5th and the 95th percentiles.[1]](/Images/godic/202501/04/Gestational_sac_diameter_by_gestational_age3520.png")
![A: Gestational sac (GS), B: Crown rump length (CRL) of embryo, C: Amniotic sac, D: Yolk sac](/Images/godic/202501/04/Gestational_sac.svg3520.png")
The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis it consists of the extra-embryonic coelom, also called the chorionic cavity. By approximately 9 weeks of gestational age the amniotic sac has expanded to occupy the majority of its volume, eventually filling the entire space. The gestational sac is usually contained with the uterus. It is the only available structure that can be used to determine if an intrauterine pregnancy exists until the embryo is identified.