Types  There are three types of gestational trophoblastic disease—hydatidiform mole (also called a molar pregnancy), invasive mole (chorioadenoma destruens) and gestational choriocarcinoma , which is further divided according to whether it is metastatic or non-metastatic.
• A hydatidiform mole develops from an abnormal embryo and contains many fluid-filled cysts. There are two
  types of hydatidiform moles, complete and incomplete (partial). The diagnosis of a complete hydatidiform mole
  is usually made during the first half of a pregnancy and is recognized by the health care provider about 50 
  percent of the time before the tumor cysts are expelled. A variety of clinical conditions may be confused with a 
  molar pregnancy, but these can usually be distinguished on the basis of medical history, a physical exam and an 
  ultrasound examination. The chance of malignant transformation is 15 to 20 percent.
      In contrast to a complete mole, a partial mole is associated with a fetus. It occurs much less frequently than a
  complete mole. The fetus usually dies within nine weeks after the last menstrual period although occasionally it
  can survive to term. Partial moles are rarely associated with multiple ovarian cysts (theca lutein cysts), high 
  HCG titers and other accompaniments of a complete mole. There is also a lower incidence of malignant behavior
  (2 to 6 percent). 
• An invasive mole (chorioadenoma destruens) is defined as a hydatidiform mole that invades the wall of the
  uterine muscle. It develops in 15 to 30 percent of all molar pregnancies.