On ultrasound, it is an anechoic (dark) space surrounded by a hyperchoic (white) rim.
It is spherical in shape, and usually located in the upper uterine fundus.
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.
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.
Characteristics on ultrasound
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).
- 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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