Gestational trophoblastic disease (GTD) is an umbrella term for a group of rare diseases in which abnormal trophoblast cells grow inside the uterus after conception. According to the National Cancer Institute, after conception, a tumor develops inside the uterus from tissue that is composed of trophoblast cells. This tissue is designed to surround the fertilized egg in the uterus, and the trophoblast cells help connect the fertilized egg to the wall of the uterus, forming part of the placenta, which supplies the embryo with nourishment from the mother.

GTD occurs after conception, when the trophoblast cells change and form a mass in the placenta, preventing a healthy fetus from developing. GTD is usually not cancerous, though some tumors may become cancerous and spread, and is generally treatable, especially if it’s caught early.