1. Complete hydatidiform moles
In complete hydatidiform moles, a sperm cell fertilizes an abnormal egg cell that has no nucleus (or chromosomes). The reason the egg contains no chromosomes is not known. After fertilization, the chromosomes from the sperm duplicate themselves, so there are 2 copies of identical chromosomes that both come from the sperm.

When this happens, normal development cannot occur, and no fetus is formed. Instead, a complete hydatidiform mole develops. Less often, a complete mole forms when an abnormal egg without any chromosomes is fertilized by 2 sperm cells. Again, there are 2 copies of the father's chromosomes and none from the mother, and no fetus forms.

2. Partial hydatidiform moles
A partial hydatidiform mole results when 2 sperm cells fertilize a normal egg at the same time. The fertilized egg contains 3 sets of chromosomes (69) instead of the usual 2 sets (46). An embryo with 3 sets of chromosomes cannot grow into a normally developed infant. Instead, this leads to an abnormal (malformed) fetus along with some normal placental tissue and a partial hydatidiform mole.

3. Invasive moles
Invasive moles are hydatidiform moles that begin to grow into the muscle layer of the uterus. They develop more often from complete moles than from partial moles. It's not clear exactly what causes this to happen.

4. Choriocarcinomas
Most choriocarcinomas develop from persistent hydatidiform moles (usually complete moles). They can also develop when bits of tissue are left behind in the uterus after a miscarriage, an intended abortion, or the delivery of a baby following an otherwise normal pregnancy. Researchers have found changes in certain genes that are commonly found in choriocarcinoma cells, but it's not clear what causes these changes.

5. Placental-site trophoblastic tumor
Placental-site trophoblastic tumor (PSTT) is an uncommon type of GTD. Unlike choriocarcinomas and hydatidiform moles, they do not have villi (tiny finger-like projections that can grow into the lining of the uterus). They develop most often after full-term pregnancies.

6. Epithelioid trophoblastic tumors
Epithelioid trophoblastic tumors are even rarer than PSTTs. Like PSTTs, they develop most often after full-term pregnancies, but it can be many years later.