Choriocarcinoma is a rare cancer that occurs as an abnormal pregnancy. A baby may or may not develop in this type of pregnancy.

The cancer may also occur after a normal pregnancy. But it most often occurs with a complete hydatidiform mole. This is a growth that forms inside the womb at the beginning of a pregnancy. The abnormal tissue from the mole can continue to grow even after attempted removal, and can become cancerous. About one half of all women with a choriocarcinoma had a hydatidiform mole, or molar pregnancy.

Choriocarcinomas may also occur after an early pregnancy that does not continue (miscarriage). They may also occur after an ectopic pregnancy or genital tumor.