Gestational sac
| Gestational sac | |
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Contents in the cavity of the uterus seen at approximately 5 weeks of gestational age by obstetric ultrasonography.
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| Details | |
| Carnegie stage | 6a |
| Days | 12 |
| Precursor | Heuser's membrane |
| Identifiers | |
| Latin | saccus gestationalis, coeloma extraembryonicum, cavitas chorionica |
| Code | TE E5.8.0.0.1.0.1 |
| Dorlands /Elsevier |
s_01/12716747 |
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Anatomical terminology
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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. The gestational sac is normally contained within the uterus. It is the only available structure that can be used to determine if an intrauterine pregnancy exists until the embryo is identified.
On obstetric ultrasound, the gestational sac is a dark ("anechoic") space surrounded by a white ("hyperechoic") rim.
The gestational sac is spherical in shape, and usually located in the upper part of the fundus of the uterus. By approximately 9 weeks of gestational age, the amniotic sac has expanded to occupy the majority of the volume of the gestational sac, eventually expanding to reduce the extraembryonic coelom to a thin layer between the amnion membrane and the mesoderm. By then, the gestational sac is usually simply called the "amniotic sac".
During embryogenesis, the extraembryonic coelom (or chorionic cavity) that constitutes the gestational sac is a portion of the conceptus consisting of a cavity between Heuser's membrane and the Trophoblast.
During formation of the primitive yolk sac, some of the migrating hypoblast cells differentiate into mesenchymal cells that fill the space between Heuser's membrane and the Trophoblast, forming the extraembryonic mesoderm. As development progresses, small lacunae begin to form within the extraembryonic mesoderm which enlarge to become the extraembryonic coelom.
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