Results from an abnormal growth of cells that are supposed to develop into the placenta. A mass develops in the uterus instead of a fetus. The exact cause is unknown but may include: defects in the egg, problems within the uterus, or nutritional deficiencies. Women under 20 or over 40 years of age have a higher risk. A molar pregnancy is also called gestational trophoblastic disease or hydatidiform mole.