Vanishing twin syndrome is a condition in which one of two fetuses disappears sometime during the first 12 weeks of a pregnancy.

Early in a multiple pregnancy, it is possible for a woman to miscarry one fetus and to notice some vaginal bleeding or to notice nothing unusual. Sometimes the embryo or fetus is reabsorbed by the mother's body.

Vanishing twin syndrome may be caused by too little of the hormone that supports pregnancy (human chorionic gonadotropin, or hCG). It does not appear to harm the mother or the surviving twin.