Hydatidiform mole, or molar pregnancy
A hydatidiform mole, also called a molar pregnancy, is a form of gestational trophoblastic disease that arises when fertilization of an egg results in an abnormal pregnancy. With a molar pregnancy, the fetus is never viable.
There are two types of molar pregnancies, complete and partial. A complete molar pregnancy develops when the fertilized egg lacks maternal genes. The pregnancy that results contains no fetal tissue and resembles grape-like cysts that fill the uterine cavity. A partial molar pregnancy occurs when more than one sperm fertilizes a normal egg and results in a pregnancy in which both the fetus and placenta are abnormal. The term partial is used because the placenta contains both normal tissue and grape-like cysts similar to that seen in complete moles.
Eighty percent of molar pregnancies are benign in that they cause no further trouble after they are removed from the uterus. However, in approximately 20 percent of complete molar pregnancies and 1-4 percent of partial moles, the molar tissue either spreads locally within the muscular wall of the uterus (called invasive mole) or spreads more widely to other parts of the body, most commonly the lungs (called metastases), requiring treatment. Hydatidiform moles occur in only one of every 1,000-1,200 pregnancies in the United States.