This is the first thing that becomes visible inside the gestational sac in an early pregnancy ultrasound. The yolk sac nourishes the developing baby (embryo) until the circulatory system has developed. The yolk sac is visible on a transvaginal ultrasound by about five weeks of gestation, but it ultimately disappears as the baby grows.
The yolk sac diameter (YSD) can be an important measurement in early pregnancy. In the first trimester, absence of the yolk sac or an unusually large yolk sac may be markers for possible impending pregnancy loss.