The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation.
It is a critical landmark, identifying a true gestation sac.
It is quite echogenic (light) to ultrasound, and reliably seen early.
A membranous sac attached to an embryo, providing early nourishment and functioning as the circulatory system of the human embryo before internal circulation begins.
Yeah, I'm bored and I can't sleep.