A molar pregnancy is an unsuccessful pregnancy, where the placenta and foetus do not form properly, and a baby does not develop.

What happens

In a normal pregnancy, the placenta provides nourishment tothe developing baby and removes waste products. The placenta is made up of millions of cells known as trophoblastic cells.

In a molar pregnancy, these cells behave abnormally as soon as the egg has been fertilised by the sperm. This results in a mass of abnormal cells that cangrow asfluid-filled sacs (cysts) with the appearance of white grapes. These cells grow rapidlywithin the womb, instead of developing into a baby. The abnormal cells are referred to as a "mole", which is from the Latin for mass or lump.

Molar pregnancy is also called a hydatidiform mole and is a pre-cancerous form of gestational trophoblastic disease.

Molar pregnancies arecaused by an imbalance in genetic material (chromosomes) in the pregnancy. This usually occurs when an egg that contains no genetic information is fertilised by a sperm (a complete molar pregnancy), or when a normal egg is fertilised by two sperm (a partial molar pregnancy).

In complete molar pregnancy, the embryo does not develop at all. In a partial molar pregnancy, a foetus can develop but never results in a viable baby, due to the imbalance between the male and female chromosomes.

A molar pregnancy is not caused by anything that you or your partner does or does not do.

How common is molar pregnancy?

Molar pregnancies are rare. About one to three in every 1,000 pregnancies turn outto be molar.

Content supplied by the NHS Website

Medically Reviewed by a doctor on 23 Jun 2016