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 moleHere, 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.
A dermoid cyst is a teratoma of a cystic nature that contains an array of developmentally mature, solid tissues. It frequently consists of skin, hair follicles, and sweat glands, while other commonly found components include clumps of long hair, pockets of sebum, blood, fat, bone, nail, teeth, eyes, cartilage, and thyroid tissue. As dermoid cysts grow slowly and contain mature tissue, this type of cystic teratoma is nearly always benign. In those rare cases wherein the dermoid cyst is malignant, a squamous cell carcinoma usually develops in adults, while infants and children usually present with an endodermal sinus tumor. : 781

Location
Due to its classification, a dermoid cyst can occur wherever a teratoma can occur. Vaginal and ovarian dermoid cysts
Ovaries normally grow cyst-like structures called follicles each month. Once an egg is released from its follicle during ovulation, follicles typically deflate. Sometimes fluid accumulates inside the follicle, forming a simple (containing only fluid) cyst. The majority of these functional cysts resolve spontaneously.While all ovarian cysts can range in size from very small to quite large, dermoid cysts are not classified as functional cysts. Dermoid cysts originate from pluripotent germ cells (which are present at birth) that differentiate abnormally, developing characteristics of mature dermal cells. Complications exist, such as torsion (twisting), rupture, and infection, although their incidence is rare. Dermoid ovarian cysts which are larger present complications which might require removal by either laparoscopy or laparotomy (traditional surgery). Rarely, a dermoid cyst can develop in the vagina.