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An anembryonic gestation (aka blighted ovum) is one in which a visible embryo never develops within a normal-appearing gestational sac, which likely occurs as a result of early embryonic death with continued development of the trophoblast. When small, the sac cannot be distinguished from the early normal pregnancy, as there may be a yolk sac, though a fetal pole is not seen. This is one of the causes of abortion of a pregnancy in the reproductive female.
For diagnosis, the sac must be of sufficient size that the absence of normal embryonic elements is established. Transvaginal ultrasound of a sac with a mean gestational sac diameter (MGD) greater than 13mm with no yolk sac, or a MGD >18mm with no embryo is sufficient to pronounce the pregnancy anembryonic. Since there is no additional risk to the mother, many centers offer the mother a follow-up study in 7-10 days to exclude error.