Gestational trophoblastic disease (GTD) is a term used for a group of pregnancy-related tumours. These tumours are rare, and they appear when cells in the womb start to proliferate uncontrollably. The cells that form gestational trophoblastic tumours are called trophoblasts and come from tissue that grows to form the placenta during pregnancy. There are several different types of GTD. A hydatidiform mole also known as a molar pregnancy, is the most common and is usually benign. Sometimes it may develop into an invasive mole, or, more rarely into a choriocarcinoma. A choriocarcinoma is likely to spread quickly, but is very sensitive to chemotherapy, and has a very good prognosis. Trophoblasts are of particular interest to cell biologists because, like cancer, they can invade tissue (the uterus), but unlike cancer, they usually "know" when to stop. GTD can simulate pregnancy, because the uterus may contain fetal tissue, albeit abnormal. This tissue may grow at the same rate as a normal pregnancy, and produces chorionic gonadotropin, a hormone which is measured to monitor fetal well-being. While GTD overwhelmingly affects women of child-bearing age, it may rarely occur in postmenopausal women. Types GTD is the common name for five closely related tumours (one benign tumour, and four malignant tumours): The benign tumour Hydatidiform mole Here, first a fertilised egg implants into the uterus, but some cells around the fetus (the chorionic villi) do not develop properly. The pregnancy is not viable, and the normal pregnancy process turns into a benign tumour. There are two subtypes of hydatidiform mole: complete hydatidiform mole, and partial hydatidiform mole. The four malignant tumours Invasive mole Choriocarcinoma Placental site trophoblastic tumour Epithelioid trophoblastic tumour All five closely related tumours develop in the placenta. All five tumours arise from trophoblast cells that form the outer layer of the blastocyst in the early development of the fetus. In a normal pregnancy, trophoblasts aid the implantation of the fertilised egg into the uterine wall. But in GTD, they develop into tumour cells. Cause Two main risk factors increase the likelihood for the development of GTD: 1) The woman being under 20 years of age, or over 35 years of age, and 2) previous GTD. Although molar pregnancies affect women of all ages, women under 16 and over 45 years of age have an increased risk of developing a molar pregnancy. Hydatidiform moles are abnormal conceptions with excessive placental development. Conception takes