The yolk sac is a small, circular structure that is the first anatomic landmark visible within the gestational sac during early pregnancy, typically seen on transvaginal ultrasound by about 5 to 6 weeks of gestation. It appears as a small, round, ring-like structure with a bright (echogenic) rim and a dark (anechoic) center. The yolk sac provides nourishment to the developing embryo before the placenta fully takes over this function and plays a role in early blood cell formation. Its presence within the gestational sac confirms an intrauterine pregnancy, which is clinically important for ruling out ectopic pregnancy. The yolk sac should measure no more than about 5 to 6 millimeters in diameter, and an abnormally large, small, or irregularly shaped yolk sac may be associated with a poor pregnancy outcome.
Sonography Term