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.
All forms of molar pregnancy, including choriocarcinoma, are more common in women of Asian or African ethnicity.
In the United States, molar pregnancies occur in about one in 1,250 pregnancies. Choriocarcinoma occurs in one in 40,000 pregnancies.