A molar pregnancy — also known as hydatidiform mole — is a noncancerous (benign) tumor that develops in the uterus. A molar pregnancy occurs when there is an extra set of paternal chromosomes in a fertilized egg. This error at the time of conception transforms what would normally become the placenta into a growing 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.

Nov. 11, 2011