Thanks for the query, The yolk sac
is a membranous sac attached to an embryo, providing early nourishment. The yolk sac appears during the 5th week. It is the second structure to appear after the GS. It should be round with an anechoic centre. It should not be calcified, misshapen or >5mm from the inner to inner diameter. Yolk sacs larger than 6 mm are usually indicative of an abnormal pregnancy
. Failure to identify (with transvaginal ultrasound
) a yolk sac when the gestational sac
has grown to 12 mm is also usually indicative of a failed pregnancy
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