A molar pregnancy refers to a pregnancy that is a type of gestational trophoblastic disease. In a molar pregnancy, the early placenta develops into a mass of cysts (called a hydatidiform mole) that resemble a bunch of white grapes. The embryo either does not form at all or is malformed and cannot survive. About one in 1,000 pregnancies is molar. Molar pregnancy poses a threat to the pregnant woman when the mole penetrates deep into the uterine wall, which can result in heavy bleeding. Occasionally, a mole can turn into a choriocarcinoma, a rare pregnancy-related form of cancer.

     After the uterus is emptied, about 20 percent of complete moles and 2 percent of partial moles persist and the remaining abnormal tissue may continue to grow. This is called persistent gestational trophoblastic disease (GTD). Treatment with one or more cancer drugs cures GTD nearly 100 percent of the time. Rarely, a cancerous form of GTD called choriocarcinoma develops that spreads to other organs. Treatment with multiple cancer drugs also is very successful at treating this cancer.