The types of gestational trophoblastic disease are hydatidiform mole, gestational trophoblastic neoplasia and placental-site trophoblastic tumour.

• Hydatidiform mole. If a woman has a hydatidiform mole (also called a molar pregnancy), the sperm and egg cells have joined without the development of a baby in the uterus. Instead, the tissue that is formed resembles grape-like cysts. A hydatidiform mole is usually benign (not cancer), but it may spread to nearby tissues (invasive mole) or become a malignant tumour called gestational trophoblastic neoplasia. Hydatidiform mole is the most common type of gestational trophoblastic disease. There are two types of hydatidiform mole: complete and partial. In a complete hydatidiform mole, there is a mass of rapidly growing abnormal cells but no fetus. In a partial hydatidiform mole, there is an abnormal nonviable fetus and placenta. The two types also differ in their genetic makeup.
• Gestational trophoblastic neoplasia. If a woman has gestational trophoblastic neoplasia, the tumour may have started from a hydatidiform mole, or from tissue that remains in the uterus following an abortion or delivery of a baby. Gestational trophoblastic neoplasia can spread from the uterus to other parts of the body.
• Placental-site trophoblastic tumour. This is a very rare type of gestational trophoblastic tumour that starts in the uterus where the placenta was attached.