• A hydatidiform mole is essentially an abnormal embryo that 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 
  molar pregnancy, but these can usually be distinguished on the basis of medical history, a physical exam and an
  ultrasound examination.
      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 (thecolutein cysts), high 
  HCG titers and other accompaniments of a complete mole. There is also a lower incidence of malignant
  behavior (5 to 10 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.
• Choriocarcinoma is a very rapidly growing malignancy that tends to spread quickly. About 50 percent of all
  cases of gestational choriocarcinoma follow a hydatidiform mole, 25 percent follow a spontaneous abortion or
  tubal pregnancy and 25 percent follow a normal pregnancy. Choriocarcinoma follows a normal-term pregnancy
  in 1 in 40,000 pregnancies. GTD after a normal pregnancy is always a choriocarcinoma, never a mole or an
  invasive mole.