Hydatidiform Mole (Molar Pregnancy)

What happens in a molar pregnancy

At fertilization, an egg and sperm normally each contribute one set of chromosomes. In a molar pregnancy, this goes wrong:

• Complete mole: an egg with no maternal genetic material is fertilized, and the paternal chromosomes duplicate. No embryo forms; the placenta grows as a mass of swollen, grape-like cysts.
• Partial mole: a normal egg is fertilized by two sperm, giving three sets of chromosomes. There may be some fetal tissue, but the fetus cannot survive.

The abnormal tissue produces large amounts of hCG, the pregnancy hormone, which explains many symptoms.

Symptoms

Many molar pregnancies are now detected on early ultrasound before symptoms develop. When present, symptoms include:

• vaginal bleeding in the first trimester, sometimes dark brown
• passing grape-like cysts
• severe nausea and vomiting (hyperemesis gravidarum)
• pelvic pressure or pain
• a uterus larger than expected for dates
• early preeclampsia, before 20 weeks, which is unusual in a normal pregnancy
• signs of an overactive thyroid, such as a racing heart and tremor, from very high hCG
• enlarged ovaries with theca lutein cysts

Risk factors

• age under 20 or over 35, particularly over 40
• a previous molar pregnancy (risk about 1 in 100 in the next pregnancy)
• previous miscarriage
• higher rates in parts of Asia

Diagnosis

• ultrasound: a complete mole may show a "snowstorm" or cystic pattern with no fetus; partial moles are harder to see
• hCG blood test: often much higher than expected
• pathology: examining the evacuated tissue under the microscope confirms the diagnosis and type

Treatment

Suction evacuation

The mole is removed by suction dilation and curettage under anesthesia. Rh-negative patients receive anti-D immunoglobulin. In people who have completed their families, hysterectomy is sometimes chosen, though follow-up is still needed.

hCG follow-up

After evacuation, hCG is measured regularly, usually weekly, until it is undetectable, then for a set period afterward (often monthly for some months for complete moles). Follow-up schedules vary; your care team will give you one. Reliable contraception is recommended during monitoring, because a new pregnancy would make hCG results impossible to interpret. Hormonal contraception is safe to use.

Persistent disease

If hCG plateaus or rises, abnormal tissue persists or has become invasive, called gestational trophoblastic neoplasia (GTN). This includes invasive mole and, rarely, choriocarcinoma. GTN is highly treatable with chemotherapy, usually methotrexate or actinomycin D for low-risk disease, with cure rates near 100 percent. See gestational trophoblastic disease.

Emotional impact

A molar pregnancy is a pregnancy loss, often combined with worry about follow-up and cancer risk. Grief, anxiety and frustration at delaying another pregnancy are common. Support from your care team, counselors and pregnancy loss groups can help.

Future pregnancies

Most people go on to have normal pregnancies. Once follow-up is complete, the risk of another molar pregnancy is about 1 percent. An early ultrasound is recommended in the next pregnancy, and an hCG check about 6 weeks after any future pregnancy ends.

Frequently asked questions

What is a hydatidiform mole?

A hydatidiform mole, or molar pregnancy, is a rare abnormal pregnancy in which placental tissue grows into a mass of fluid-filled cysts instead of a normal pregnancy. It results from a genetic error at fertilization.

What are the symptoms of a molar pregnancy?

Symptoms include vaginal bleeding in early pregnancy, passing grape-like cysts, severe nausea and vomiting, pelvic pressure, a uterus larger than expected, and very high hCG. Many are found on a routine early ultrasound.

How is a molar pregnancy treated?

The tissue is removed with suction dilation and curettage. Afterward, hCG blood tests are done regularly until levels return to zero, to confirm no abnormal tissue remains. A small number of people need chemotherapy, which is highly effective.

Can a molar pregnancy turn into cancer?

Occasionally. About 15 to 20 percent of complete moles and a small percentage of partial moles leave persistent tissue that needs treatment. Rarely, this is choriocarcinoma. These conditions are almost always curable with chemotherapy.