THE NORMAL YOLK SAC (SECONDARY) The primary yolk sac develops due to growth of the extra-embryonic ectoderm from the ventral aspect of the embryonic disc. It cannot be visualized sonographically and soon degenerates and is replaced by the secondary yolk sac. First structure visible within the gestational sac and is of embryonic origin. The appearance of a yolk sac within the gestational sac excludes a blighted ovum pregnancy and ectopic pregnancy (rare cases of heterotopic pregnancies are however not excluded). Always seen when gestational sac = 13mm. Seen at 5 weeks MA when sac = 4mm in diameter. Confirms that the fluid collection within the uterus is a pregnancy as the double decidual reaction is not 100% specific for the presence of an intrauterine pregnancy. ULTRASOUND Round or spherical with a bright echogenic rim and internal anechoic area. It is localized outside the amnion within the chorionic space. 4-7 mm between 5 - 10 weeks GA Mean yolk sac diameter during the first trimester Gestational age (weeks) Sonographic diameter (mm ±SD) Growth = 0.1mm per millimeter growth of MSD when MSD
Posted on: Fri, 03 Oct 2014 22:03:30 +0000
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