Molar pregnancy

Alternative names
Hydatid mole; Hydatidiform mole

A hydatidiform mole is a rare mass or growth that may form inside the uterus at the beginning of a pregnancy. See also choriocarcinoma.

Causes, incidence, and risk factors

A hydatidiform mole results from over-production of the tissue that is supposed to develop into the placenta. The placenta normally nourishes a fetus during pregnancy. Instead, these tissues develop into a mass. The mass is usually made up of placental material that grows uncontrolled. Often, there is no fetus at all.

The cause is not completely understood. Potential causes may include defects in the egg, abnormalities within the uterus, or nutritional deficiencies. Women under 20 or over 40 years of age have a higher risk. Other risk factors include diets low in protein, folic acid, and carotene.


  • Vaginal bleeding in pregnancy during the first trimester  
  • Nausea and vomiting, severe enough to require hospitalization in 10% of cases  
  • An abnormal growth in the size of the uterus, for the stage of the pregnancy       o Excessive growth in approximately 1/2 of cases       o Smaller-than-expected growth in approximately 1/3 of cases  
  • Symptoms of hyperthyroidism:       o Rapid heart rate       o Restlessness, nervousness       o Heat intolerance       o Unexplained weight loss       o Loose stools       o Trembling hands       o Skin warmer and more moist than usual  
  • Symptoms similiar to preeclampsia that occur in the 1st trimester or early in the 2nd trimester. (This almost always indicates hydatidiform mole, because preeclampsia is extremely rare this early in normal pregnancies.)       o High blood pressure       o Swelling in feet, ankles, legs       o Proteinuria

Note: All symptoms occur in conjunction with a potential, suspected, or confirmed pregnancy.

Signs and tests

A pelvic examination may show signs similar to a normal pregnancy, but the uterine size may be abnormal and fetal heart tones are absent. Additionally, some bleeding may be noted.

Tests typically include:

  • Measurement of serum HCG to confirm pregnancy. Repeated HCG measurements can be used to monitor the rate and consistency of decline if a hydatidiform mole is suspected.  
  • An ultrasound of the pelvis.  
  • A chest X-ray and abdominal CT or MRI will be recommended for some patients.

This disease may also alter the results of the following tests:

  • Transvaginal ultrasound  
  • HCG (quantitative)


If a miscarriage does not occur and the diagnosis is confirmed, a therapeutic abortion is performed by suction curettage (D and C).

Following either case, serum HCG levels are monitored to assure they return to a normal, non-pregnant level. A hysterectomy may be an option for older women who do not desire future pregnancies.

Expectations (prognosis)

More than 80% of hydatidiform moles are benign (non-cancerous). The outcome after treatment is usually excellent. Close follow-up is essential. Highly effective means of contraception are recommended to avoid pregnancy for at least 6-12 months.

In 10-15% of cases, hydatidiform moles may develop into invasive moles. These may intrude so far into the uterine wall that hemorrhage or other complications develop.

In 2-3% of cases, hydatidiform moles may develop into choriocarcinoma, which is a malignant, rapidly growing, and metastatic (spreading) form of cancer. Despite these factors, which normally indicate a poor prognosis, the rate of cure after treatment with chemotherapy is high.

Over 90% of women with malignant, non-spreading cancer are able to survive and retain their ability to have children. In those with metastatic (spreading) cancer, remission remains at 75-85%, although the ability to have children is usually lost.

Lung insufficiency may develop after evacuation of the uterus in cases where the uterus enlarges to greater than 16 weeks gestational size.

Calling your health care provider

Call your obstetrician if you suspect a hydatidiform mole.

If you have symptoms suggestive of preeclampsia - such as severe swelling in the legs and feet, abdominal pain and high blood pressure - see your health care provider immediately, call 911, or get to the emergency room. This can rapidly become a life-threatening emergency.

Adequate nutrition may reduce the risk.

Johns Hopkins patient information

Last revised: December 3, 2012
by Levon Ter-Markosyan, D.M.D.

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