What is "molar pregnancy"

n. an abnormality during pregnancy; chorionic villi around an aborting embryo degenerate and form clusters of fluid-filled sacs [syn: hydatid mole, hydatidiform mole]

Molar pregnancy is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus and will fail to come to term. A molar pregnancy is a gestational trophoblastic disease which grows into a mass in the uterus that has swollen chorionic villi. These villi grow in clusters that resemble grapes. A molar pregnancy can develop when a fertilized egg does not contain an original maternal nucleus. The products of conception may or may not contain fetal tissue. It is characterized by the presence of a hydatidiform mole (or hydatid mole, mola hydatidosa). Molar pregnancies are categorized as partial moles or complete moles, with the word mole, being used to denote simply a clump of growing tissue, or a growth.

A complete mole is caused by a single (incidence is about 90%) or two (incidence is about 10%) sperm combining with an egg which has lost its DNA (the sperm then reduplicates forming a "complete" 46 chromosome set) The genotype is typically 46,XX ( diploid) due to subsequent mitosis of the fertilizing sperm, but can also be 46,XY (diploid). 46,YY (diploid) is not observed. In contrast, a partial mole occurs when a haploid egg is fertilized by two sperm or by one sperm which reduplicates itself yielding the genotypes of 69,XXY ( triploid) or 92,XXXY ( tetraploid). Complete hydatidiform moles have 2.5% risk of developing into choriocarcinoma.

The etymology is derived from hydatisia ( Greek "a drop of water"), referring to the watery contents of the cysts, and mole (from Latin mola = millstone/false conception). The term, however, comes from the similar appearance of the cyst to a hydatid cyst in an Echinococcosis.