After a sperm fertilizes an egg, new tissues develop that normally form the fetus and placenta. A molar pregnancy, also known as gestational trophoblastic disease, occurs when the tissue that was supposed to form the placenta grows abnormally and can form a tumor that can spread beyond the womb or uterus.
In a "complete mole," no normal fetal tissue forms. In a "partial mole," incomplete fetal tissues develop alongside molar tissue. These two conditions are noncancerous (benign) and make up 80 percent of cases. Three malignant forms of gestational trophoblastic disease occur, including invasive molar pregnancy, choriocarcinoma and placental site trophoblastic tumors. Almost all molar pregnancies, even the cancerous type, can be cured.
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