A hydatidiform mole is growth of an abnormal fertilized egg or an overgrowth of tissue from the placenta.
Most often, a hydatidiform mole is an abnormal fertilized egg that develops into a hydatidiform mole rather than a fetus (a condition called molar pregnancy). However, a hydatidiform mole can develop from cells that remain in the uterus after a miscarriage or a full-term pregnancy. Rarely, a hydatidiform mole develops when there is a living fetus. In such cases, the fetus typically dies, and a miscarriage often occurs.
Hydatidiform moles are most common among women under 17 or over 35. In the United States, they occur in about 1 in 2000 pregnancies in the United States. For unknown reasons, moles are almost 10 times more common in Asian countries.
About 80% of hydatidiform moles are not cancerous. About 15 to 20% invade the surrounding tissue and tend to persist. About 2 to 3% become cancerous and spread throughout the body. They are then called choriocarcinomas. Choriocarcinomas can spread quickly through the lymphatic vessels or bloodstream. Hydatidiform moles and choriocarcinomas are types of gestational trophoblastic disease.
Women who have a hydatidiform mole feel as if they are pregnant. But because hydatidiform moles grow much faster than a fetus, the abdomen becomes larger much faster than it does in a normal pregnancy. Severe nausea and vomiting are common, and vaginal bleeding may occur. As parts of the mole deteriorate, small amounts of tissue, which resemble a bunch of grapes, may pass through the vagina. These symptoms indicate the need for prompt evaluation by a doctor.
Hydatidiform moles can cause serious complications, including infections and very high blood pressure with increased protein in the urine (preeclampsia or eclampsia—see Complications of Pregnancy: Preeclampsia).
If choriocarcinoma develops, women may have other symptoms, caused by spread (metastasis) to other parts of the body.
Often, doctors can diagnose a hydatidiform mole shortly after conception. The pregnancy test is positive, but no fetal movement and no fetal heartbeat are detected, and the uterus is much larger than expected.
Ultrasonography is done to be sure that the growth is a hydatidiform mole and not a fetus or amniotic sac (which contains the fetus and fluid around it). Blood tests to measure the level of human chorionic gonadotropin (hCG—a hormone normally produced early in pregnancy) are done. If a hydatidiform mole is present, the level is usually very high because the mole produces a large amount of this hormone. A sample of tissue is removed or obtained when it is passed, then examined under a microscope (biopsy) to confirm the diagnosis.
The cure rate for a hydatidiform mole is virtually 100% if the mole has not spread. The cure rate is 60 to 80% for choriocarcinoma that has spread widely. Most women can have children afterwards and do not have a higher risk of having complications, a miscarriage, or children with birth defects.
About 1% of women who have had a hydatidiform mole have another one. So if women have had a hydatidiform mole, ultrasonography is done early in subsequent pregnancies.
A hydatidiform mole is completely removed, usually by dilation and curettage (D and C) with suction (see Diagnosis of Gynecologic Disorders: Dilation and Curettage). Only rarely is removal of the uterus (hysterectomy) necessary.
A chest x-ray is done to see whether the mole has become cancerous (that is, a choriocarcinoma) and spread to the lungs. After surgery, the level of human chorionic gonadotropin in the blood is measured to determine whether the hydatidiform mole was completely removed. When removal is complete, the level returns to normal, usually within 10 weeks, and remains normal. If the level does not return to normal (called persistent disease), computed tomography (CT) of the brain, chest, abdomen, and pelvis is done to determine whether choriocarcinoma has developed and spread.
Hydatidiform moles do not require chemotherapy, but persistent disease does. Usually, only one drug (methotrexate or dactinomycin) is needed. Sometimes both drugs or another combination of chemotherapy drugs is needed.
Women who have had a hydatidiform mole removed are advised not to become pregnant for 1 year. Oral contraceptives are frequently recommended, but other effective contraceptive methods can be used.
Last full review/revision November 2008 by David M. Gershenson, MD; Pedro T. Ramirez, MD