Yolk sac
Small circular structure visible on early ultrasound within the gestational sac, important for early pregnancy assessment.
Full Definition
The yolk sac is a small, round, echogenic structure that appears within the gestational sac around 5-6 weeks of pregnancy and serves as the primary source of nutrition for the early embryo before placental circulation is established. Sonographically, it appears as a thin-walled circular structure measuring 2-5mm in diameter. The yolk sac should be visible when the gestational sac reaches 8-10mm and its presence confirms an intrauterine pregnancy. Abnormal yolk sac characteristics, such as irregular shape, enlarged size (>6mm), or absence when expected, may indicate embryonic abnormalities or impending pregnancy failure.
Usage
Usage note: Always written as two words. Critical landmark for confirming intrauterine pregnancy and embryonic development.
In Context
- "A normal-appearing yolk sac measuring 3.2mm was identified within the gestational sac at 6 weeks." — Early pregnancy ultrasound
- "The enlarged yolk sac measuring 7mm raised concern for possible chromosomal abnormality." — First trimester assessment