Yolk sac is the 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 hematopoiesis, providing endocrine, metabolic and immunological functions and contributing to the development of fetal gastrointestinal and reproductive systems .
As the pregnancy advances, the yolk sac progressively increases from the 5 to end of the 10 gestational week, following which the yolk sac gradually disappears and is often sonographically undetectable after 14-20 weeks.
- yolk sac appears as a circular thick walled echogenic structure with an anechoic center within the gestational sac, but outside the amniotic membrane
- when at 5.0 to 5.5 weeks, it can sometimes be seen as two parallel lines rather than a discrete circle
- yolk sac should be seen on transabdominal scanning when the mean sac diameter (MSD) is 20 mm or at a gestational age of 7 weeks and is usually seen endovaginally with an MSD of 8-10 mm or gestational age of 5.5 weeks.
- visualization of a yolk sac is useful in distinguishing an intrauterine pregnancy (IUP) from a pseudogestational sac, a decidual cast cyst or an anembryonic pregnancy, as a yolk sac is only seen in an intrauterine pregnancy
In a normal early pregnancy, the diameter of the yolk sac should usually be <6 mm while its shape should be near spherical. A yolk sac ≥6 mm is suspicious for a failed pregnancy, but not diagnostic.
Visualization of multiple yolk sacs is the earliest sign of a polyamniotic pregnancy, e.g. twins (see case 4). The number of yolk sacs matches the number of amniotic sacs if the embryos are alive .