A rare condition in which abnormal cells grow inside the uterus from tissue that forms after conception (the joining of sperm and egg). This tissue is made of trophoblastic cells, which normally surround the fertilized egg in the uterus and help connect the fertilized egg to the wall of the uterus. These cells also form part of the placenta (the organ that passes nutrients from the mother to the fetus). Most GTTs are benign (not cancer) and do not spread, but some types are malignant (cancer) and spread to nearby tissues or other parts of the body. The two main types of GTTs are hydatidiform mole and choriocarcinoma. Also called gestational trophoblastic disease, gestational trophoblastic tumor, and GTD.