الفهرس 
Embryology of fetus and yolk sac
Important aspects of specific fetal systems developmentOnly 14 pages are availabe for public view
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Abstract
The yolk sac is the first conceptional structure that becomes sonographically visible within the gestational sac. It is essential in early embryonic life.Specifically, the yolk sac provides nutritional, endocrine, metabolic, immunologic, and hematopoietic functions during organogenesis until establishment of the placental circulation.Normally, the yolk sac is a round structure with an anechoic center that is surrounded by a uniformly thick and well-defined echogenic wall (79).The yolk sac can often be shown on sonography when the mean gestational sac diameter is 5 to 6 mm, and it can be detected before visualization of the embryo.Although there is no consensus about the upper normal limit of the YSD, previous studies claimed that pregnancies with a YSD larger than 5 mm had an increased risk of spontaneous abortion and therefore recommended close follow-up with sonography for these pregnancies