A blighted ovum is a type of early pregnancy loss where a fertilized egg implants in the uterus but doesn't develop into an embryo. Another name for a blighted ovum is empty sac, or anembryonic pregnancy. The mother will develop a placenta and a gestational sac, but the embryo does not grow. In some cases a blighted ovum may occur so early that the mother does not even know she is pregnant. In other cases the placenta and sac will give off pregnancy hormones, which will allow a pregnancy test to still show positive and the mother to experience symptoms of pregnancy.