An anembryonic gestation (also known as a blighted ovum) is a pregnancy in which the very early pregnancy appears normal on an ultrasound scan, but as the pregnancy progresses a visible embryo never develops. In a normal pregnancy, an embryo would be visible on an ultrasound by six weeks after the woman's last menstrual period.
An anembryonic gestation is characterized by a normal-appearing gestational sac, but the absence of an embryo. It 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. For diagnosis, the sac must be of sufficient size that the absence of normal embryonic elements is established. The criteria depends on the type of ultrasound exam performed. A pregnancy is anembryonic if a transvaginal ultrasound reveals a sac with a mean gestational sac diameter (MGD) greater than 13 mm and no yolk sac, or an MGD >18 mm with no embryo. If a transvaginal exam is not performed, the criteria for a transabdominal scan is a MGD of 25 mm or more without an embryo or an MGD of 20 mm or more without a yolk sac.
Rather than do a transvaginal exam at the time of the initial visit, many centers prefer to do only a transabdominal study and offer mothers a follow-up ultrasound 10 days later to see if a normal pregnancy subsequently develops.
Anembryonic gestation is one of the causes of miscarriage of a pregnancy. Several management options exist for anembryonic pregnancies which have not miscarried on their own.