After a sperm fertilizes an egg, new tissues develop that normally form the fetus and placenta. A molar pregnancy, also known as gestational trophoblastic disease, occurs when the tissue that was supposed to form the placenta grows abnormally and can form a tumor that can spread beyond the womb or uterus.
In a “complete mole,” no normal fetal tissue forms. In a “partial mole,” incomplete fetal tissues develop alongside molar tissue. These two conditions are noncancerous (benign) and make up 80 percent of cases. Three malignant forms of gestational trophoblastic disease occur, including invasive molar pregnancy, choriocarcinoma and placental site trophoblastic tumors. Almost all molar pregnancies, even the cancerous type, can be cured.
Most molar pregnancies are noncancerous and confined to the uterus (hydatidiform moles). In this type of mole, the abnormal placental tissue has villi, clusters of tissue swollen with fluid, giving it the appearance of a cluster of grapes. If a fetus begins to develop along with a hydatidiform mole, it typically has many malformations and almost never can be delivered as a living baby.
A more aggressive tumor associated with molar pregnancies is an invasive mole, also called chorioadenoma destruens. The invasive mole contains many villi, but these may grow into or through the muscle layer of the uterus wall. Rarely, invasive moles can cause bleeding by perforating the uterus through its whole thickness. In 15% of cases, an invasive mole can spread to tissues outside of the uterus.
Pregnancy tissues can develop into a cancer called choriocarcinoma, though this is rare. Fifty percent of choriocarcinomas form during a molar pregnancy. Others form during a tubal pregnancy, an aborted pregnancy, a miscarriage or a healthy pregnancy. Choriocarcinomas can cause persistent bleeding in the weeks or months after delivery, but this happens very rarely. (Most bleeding like this is not caused by a choriocarcinoma). Choriocarcinomas associated with molar pregnancies almost always follow complete moles rather than partial moles.
In the United States, molar pregnancies occur in about one in 1,200 pregnancies. Choriocarcinoma occurs in one in 40,000 pregnancies.
Hydatidiform moles can exaggerate the usual symptoms of pregnancy. Many of the symptoms are similar to those associated with miscarriage, and most women with molar pregnancies first believe they have miscarried. Invasive moles and choriocarcinomas can cause symptoms during or after pregnancy, and symptoms can develop after a hydatidiform mole has been removed.
The most common symptom is vaginal bleeding, especially between the 6th and 16th weeks of pregnancy. Another symptom is bleeding that continues for a long time after delivery. Small amounts of bleeding can show up as a watery brown discharge from the vagina. Sometimes, a piece of tissue containing grapelike shapes will pass through the vagina, though this is not common. It is important to remember that most vaginal bleeding during or after pregnancy is not associated with a molar pregnancy. However, you should report any bleeding during pregnancy to your health care professional.
A mole or choriocarcinoma also can cause the following symptoms:
- Abdominal swelling, caused by the uterus becoming larger, that occurs more rapidly than expected for the first trimester of pregnancy
- Excessive vomiting during pregnancy
- Fatigue, often caused by anemia from heavy bleeding
- Sudden severe abdominal pain caused by internal bleeding
- Pelvic cramping or vaginal discharge
- Shortness of breath, coughing or blood in coughed-up secretions because choriocarcinoma very rarely spreads to the lungs before it is diagnosed.
There are many other causes for these symptoms—most are associated with normal pregnancies—so if you have such problems don't assume you have a molar pregnancy. Always speak with your health care professional.
Your doctor may suspect you have a molar pregnancy based on symptoms you have during or following pregnancy, or because your uterus is unusually large. Your doctor may suspect a molar pregnancy if you have a high level of human chorionic gonadotropin (HCG), the hormone measured in a routine pregnancy test. All pregnancies with high levels of HCG are not moles, however, and some molar pregnancies do not have high levels of HCG.
An ultrasound of the pelvis typically can confirm a diagnosis of a molar pregnancy. Ultrasound uses sound waves to show an image of the contents within the uterus.
If you have a molar pregnancy, further testing will be done to determine the type of mole and the possibility of it having spread outside the uterus. Testing may include X-rays, computed tomography (CT) scans or magnetic resonance imaging (MRI) scans to view the chest, abdomen, pelvis and brain. Additional blood tests may be needed. A pathologist will look at the molar tissue under a microscope once it is removed to confirm the diagnosis.
With appropriate treatment, all hydatidiform moles are curable, and nearly all cases of more aggressive molar tumors can be cured. Even with tumors whose features categorize them as having a poor prognosis, 80% to 90% are cured with a combination of surgery and, if needed, chemotherapy.
It is important for women with molar pregnancies to be evaluated periodically after the problem has been treated. Women are advised not to attempt pregnancy for some time in order to be sure that levels of HCG remain at zero and that no further treatment is needed. There is a risk that a molar pregnancy can come back after treatment. Recommendations are changing and vary by hospital.
It is usually possible for women to have a normal, healthy pregnancy after treatment for a molar pregnancy.
Treatment for some molar pregnancies can take several months. Following treatment, you will need to have repeated blood tests and checkups over one to two years, to be sure that all molar tissue has been treated and that the problem has not returned.
The results of diagnostic tests will help to determine a treatment plan. Options for treatment almost always include surgery to remove the tumor. More aggressive types of molar pregnancy may require chemotherapy and/or radiation therapy. About 85% of hydatidiform moles can be treated without chemotherapy. Treatment options include:
- Suction dilation and curettage (D and C) — This is a surgical procedure used to remove noncancerous hydatidiform moles. The opening in the cervix is dilated and the inside uterus lining is scraped (curetted) clean using suction and another spoon-like instrument.
- Removal of the uterus (hysterectomy) — This is used rarely to treat hydatidiform moles but may be elected particularly if the woman does not want to become pregnant again.
- Chemotherapy with a single drug — This treatment with medication toxic to the molar tissue is used to treat a molar pregnancy tumor that has features suggesting a good prognosis.
- Chemotherapy with multiple drugs — Treatment with several medications, each toxic to the molar tissue, is generally required to treat invasive tumors with poorer prognosis.
- Radiation treatment — This uses high-strength X-ray beams to destroy cancer cells in the exceedingly rare case when a tumor has spread (metastasized) to the brain.
Although any woman who becomes pregnant is at some risk of developing one of these uncommon conditions, the risk appears to be higher in pregnant women who are younger than age 20 or older than age 40.
The best way to prevent complications from an invasive mole or a choriocarcinoma is to receive routine prenatal care by a qualified health care professional, so that problems can be identified as early as possible.
When to Call a Professional
For any pregnancy, make sure you have appropriate prenatal care beginning in the first trimester, with regular checkups. Report any bleeding, excessive vomiting, or abdominal pain during pregnancy to your doctor. If you have prolonged vaginal bleeding after childbirth, an abortion or a miscarriage, contact your doctor for an evaluation.