A cancerous (malignant) tumour that starts in the layer of the uterus (womb) that normally develops into the placenta during pregnancy.

Choriocarcinoma is the cancerous (malignant) form of gestational trophoblastic disease. On rare occasions, choriocarcinoma may develop in a testicle or ovary (these tumours are not the same as gestational choriocarcinoma).

Compare with hydatidiform mole.