Tumors can grow anywhere in the body and happen when cells in the body begin to grow out of control. Some tumors might have cancer cells within them, and some might not.  Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. 

Gestational trophoblastic disease (GTD) is a group of rare tumors that involve abnormal growth of cells inside a woman's uterus. GTD does not develop from cells of the uterus like cervical cancer or endometrial (uterine lining) cancer do. Instead, these tumors start in the cells that would normally develop into the placenta during pregnancy.  

GTD begins in the layer of cells called the trophoblast that normally surrounds an embryo. (Tropho- means nutrition, and -blast means bud or early developmental cell.) Early in normal development, the cells of the trophoblast form tiny, finger-like projections known as villi. The villi grow into the lining of the uterus. In time, the trophoblast layer develops into the placenta, the organ that protects and nourishes the growing fetus.

Most GTDs are benign (not cancer) and they don't invade deeply into body tissues or spread to other parts of the body. But some are malignant (cancerous). Because not all of these tumors are cancerous, this group of tumors may be referred to as gestational trophoblastic disease, gestational trophoblastic tumors, or gestational trophoblastic neoplasia.