yolk sac
The yolk sac is a temporary, membranous structure that provides nutrients and red blood cells to a developing embryo in early pregnancy before the placenta takes over. It appears around the fourth week of gestation, is visible on ultrasound from about week five, and is absorbed by the embryo after week 10, disappearing completely by the second trimester. A healthy yolk sac is round or pear-shaped, and its appearance and size on an early ultrasound are used to assess the viability of a pregnancy.
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