What is the yolk sac?

The yolk sac is a membraneous sac attached to the embryo and is the first anatomical structure identified within the gestational sac. The yolk sac plays a critical role in embryonal development by providing nutrients, serving as the site of initial blood creation (haematopoiesis), providing hormones, 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 5th to end of the 10th gestational week, following which the yolk sac gradually disappears and is often sonographically undetectable after 14-20 weeks.

 

Early pregnancy measurements yolk sac gestational sac crl

From: fetalMedicine