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NCI: The sac that envelops the embryo. It is composed of extraembryonic membranes.,MSH: Round-shaped structure usually located in the upper fundus of the UTERUS in early pregnancy. It comprises EMBRYO; AMNIOTIC FLUID; and YOLK SAC. It is visible in PRENATAL ULTRASONOGRAPHY by the fifth week of pregnancy. Its size is often used to determine and monitor GESTATIONAL AGE; FETAL GROWTH; and PREGNANCY COMPLICATIONS.
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