Decorative image

Risks and causes

A molar pregnancy occurs when the fertilisation of the egg by the sperm goes wrong and leads to the growth of abnormal cells or clusters of water filled sacs inside the womb. 

These tumours are rare. In the UK, there is about 1 molar pregnancy for every 590 pregnancies.

Molar pregnancies are caused 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).

Risk factors for molar pregnancy

A risk factor is anything that can increase your chance of developing a disease or condition. Each condition has different risk factors. Researchers haven't yet been able to identify what causes molar pregnancies. But we do know of some factors that increase a woman's risk of developing one.

Remember that having a risk factor doesn't mean that you will definitely get a particular medical condition. Most women who have one or more risk factors never develop a molar pregnancy, and some people who have none of the risk factors do develop one. This is only a guide to what might increase risk.


All women who become pregnant have a risk of developing a molar pregnancy but the risk is very small. Researchers have found that some types of molar pregnancy are more common in certain age groups.

Complete molar pregnancies are more common in mothers over the age of 45. There is a moderate increased risk for young teenagers.

Age doesn't affect the risk of partial molar pregnancies.

Previous molar pregnancy

If you've already had one molar pregnancy, you have about a 1 in 100 chance (1%) of having another one. This means that 99 out of every 100 women (99%) who become pregnant after a molar pregnancy will have a normal pregnancy.

If you have had two or more molar pregnancies your risk of having another is higher, at about 15 to 20 out of 100 (15 to 20%).