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Yolk sac

Dr Abhilash Sandhyala and Dr Frank Gaillard et al.

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.

Radiographic assessment

Ultrasound

Visualisation of a yolk sac is useful in distinguishing an intrauterine pregnancy (IUP) from a pseudogestational sac, a decidual cast cyst or a blighted ovum, as it is only seen in the former.

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.