A yolk sac is first anatomical structure identified within the gestational sac. It plays a critical role in embryonal development by providing nutrients, serving as the site of initial haematopoiesis and contributing to the development of gastrointestinal and reproductive systems 2.
A yolk sac should always be seen when the mean sac diameter (MSD) is 20 mm on trans-abdominal scanning and usually seen trans-vaginally with an MSD of 8 - 10 mm.
In general if the MSD is 16 mm or greater and no fetal pole / yolk sac can be identified on trans-vaginal scanning then this suggests a non-viable pregnancy (anembryonic pregnancy). Repeat scanning with an larger MSD and serial quantitative beta-HCGs is however thought prudent.
In a normal early pregnancy, the diameter of the yolk sac should usually be < 6 mm while its shape should be near spherical.
Visualisation multiple yolk sacs is the earliest sign of a polyamniotic pregnancy, e.g twins.
As the pregnancy advances, the yolk sac disappears and is often sonographically not detectable after 14 weeks.
- 1. N Gupta and TL Angtuaco "Embryosonology in the First Trimester of Pregnancy" Ultrasound Clinics 2 (2007) 175-185
- 2. Callen PW. Ultrasonography in obstetrics and gynecology. W B Saunders Co. (2008) ISBN:1416032649. Read it at Google Books - Find it at Amazon
- 3. Chudleigh P, Thilaganathan B. Obstetric ultrasound, how, why and when. Churchill Livingstone. (2004) ISBN:0443054711. Read it at Google Books - Find it at Amazon
- 4. Lindsay DJ, Lovett IS, Lyons EA et-al. Yolk sac diameter and shape at endovaginal US: predictors of pregnancy outcome in the first trimester. Radiology. 1992;183 (1): 115-8. Radiology (abstract) - Pubmed citation
- 5. Küçük T, Duru NK, Yenen MC et-al. Yolk sac size and shape as predictors of poor pregnancy outcome. J Perinat Med. 1999;27 (4): 316-20. doi:10.1515/JPM.1999.045 - Pubmed citation
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