Risks and causes of molar pregnancy

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. This condition is a type of gestational trophoblastic tumour. The link takes you to general information about what these tumours are.

A risk factor is anything that can increase your chance of developing a disease or condition. Each condition has different risk factors. Researchers have not yet been able to identify what causes molar pregnancies or gestational trophoblastic tumours (GTT), but we do know of some factors that increase a woman's risk of developing this type of tumour.

Remember that having a risk factor does not 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 may increase risk. Gestational trophoblastic tumours are rare. In the UK, there is about 1 molar pregnancy for every 1,000 pregnancies - that is about 0.1%. Or 1 molar pregnancy for every 750 babies born.

Molar pregnancy is an abnormal pregnancy, so all women who become pregnant have a slight risk of developing it. However, researchers have found that some types of molar pregnancy are more common in certain age groups. Complete molar pregnancies are more common in teenage mothers and mothers over the age of 45. For mothers over the age of 50 the risk of molar pregnancy is around 1 in 4 (25%). Age plays less of a risk in partial molar pregnancies but the risk is still higher in younger mothers and women over the age of 45.

Molar pregnancies used to be much more common in women from Asian countries than in those from other ethnic groups. However, recent studies show that in Asia, and in Asian women living in the UK, the incidence is now about 1 molar pregnancy for every 380 babies born, compared to around 1 for every 750 babies born in the UK overall.

If you have already had one molar pregnancy, you have about a 1 in 100 chance (1%) of having another one. This means that more than 98 out of every 100 women (98%) 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 1 in 10 (10%).

In the past doctors were concerned that taking the oral contraceptive pill for a long time may increase your risk of developing a molar pregnancy. Results of studies have varied and now generally researchers don't think that the pill increases the risk of molar pregnancy. But if you have had a molar pregnancy, your doctor will advise you not to take the pill until your urine and blood hCG tests go back to normal. This is because some studies have shown that using the contraceptive pill before the molar pregnancy has completely gone may mean that you are more likely to need chemotherapy.