A blighted ovum is a pregnancy in which the embryo is reabsorbed or never develops at all. In a normal pregnancy, an embryo would be visible on an ultrasound by six weeks after the woman's last menstrual period. A blighted ovum cannot result in a viable pregnancy. Because of the presence of hCG, an ultrasound is typically necessary to diagnose an anembryonic pregnancy. Generally, a transvaginal ultrasound is used to investigate a suspected case of blighted ovum. A transabdominal ultrasound can be used to diagnose anembryonic pregnancy if a gestational sac can be identified, but is empty.