|Latin||saccus gestationalis, coeloma extraembryonicum, cavitas chorionica|
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 the only available intrauterine structure that can be used to determine if an intrauterine pregnancy (IUP) exists, until the embryo is identified.
On obstetric ultrasound, the gestational sac is an anechoic (dark) space surrounded by a hyperchoic (white) rim.
The yolk sac and embryo should be readily identified when the gestational sac reaches a certain size—a yolk sac should be seen when gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm.
Gestational sacs can be identified via ultrasound and are generally identified by the following 4 characteristics:
- Round or elliptical shape in longitudinal and transverse views
- Surrounded by an echogenic rim (choriodecidual reaction)
- Gestational sac is in uterine fundus
- Sac is not directly midline, but implanted eccentrically (to one side of the uterine cavity line, without displacing it).
Anatomy and embryology
The extraembryonic coelom (or chorionic cavity) that constitutes the gestational sac for most part of the embryogenesis 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 transdifferentiate 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 become larger and form the extraembryonic coelom.
The extraembryonic coelom divides the extraembryonic mesoderm into two layers: extraembryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac, and extraembryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.
The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells.
It is spherical in shape, and usually located in the upper uterine fundus.
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".
Below are useful terms on ultrasound:
- Echogenic - giving rise to reflections (echoes) of ultrasound waves
- Hyperechoic – more echogenic (brighter) than normal
- Hypoechoic – less echogenic (darker) than normal
- Isoechoic – the same echogenicity as another tissue
- Transvaginal ultrasonography - Ultrasound is performed through the vagina
- Transabdominal ultrasonography - Ultrasound is performed across the abdominal wall or through the abdominal cavity
In normal state, each body tissue type, such as liver, spleen or kidney, has a unique echogenicity. Fortunately, gestational sac, yolk sac and embryo are surrounded by hyperechoic (brighter) body tissues.
- Papaioannou, George I.; Syngelaki, Argyro; Poon, Leona C.Y.; Ross, Jackie A.; Nicolaides, Kypros H. (2010). "Normal Ranges of Embryonic Length, Embryonic Heart Rate, Gestational Sac Diameter and Yolk Sac Diameter at 6–10 Weeks". Fetal Diagnosis and Therapy 28 (4): 207–219. doi:10.1159/000319589. ISSN 1421-9964.
Crown rump length before a gestational age of 6 weeks is extrapolated by assuming a gestational sac diameter of 0 at a gestational age of 2 weeks, corresponding to a fertilization age of 0 weeks. This extrapolated part is blurred because of uncertain values.
- Karki DB, Sharmqa UK, Rauniyar RK (2006). "Study of accuracy of commonly used fetal parameters for estimation of gestational age". JNMA; journal of the Nepal Medical Association 45 (162): 233–7. PMID 17189967.
- "Basic Imaging > Ultrasound of Early Pregnancy". Archived from the original on 2007-08-14. Retrieved 2007-10-13.
- Zwingenberger, Allison (10 April 2007). "What do hyperechoic and hypoechoic mean?". DVM Journals.
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