A molar pregnancy — also known as hydatidiform mole — is a noncancerous (benign) tumor that develops in the uterus. A molar pregnancy starts when an egg is fertilized, but instead of a normal, viable pregnancy resulting, the placenta develops into an abnormal mass of cysts.
In a complete molar pregnancy, there's no embryo or normal placental tissue. In a partial molar pregnancy, there's an abnormal embryo and possibly some normal placental tissue. The embryo begins to develop but is malformed and can't survive.
A molar pregnancy can have serious complications — including a rare form of cancer — and requires early treatment.