What is molar pregnancy?

Molar pregnancy is one of a group of conditions known as gestational trophoblastic disease (GTD). It is sometimes called a hydatidiform mole.

It happens when the fertilisation of the egg by the sperm goes wrong. This leads to the growth of abnormal cells or clusters of water filled sacs inside the womb. Unfortunately, a molar pregnancy cannot develop into a healthy baby.

Molar pregnancies are not cancer (they are benign). But there is a very small risk that the molar cells could become cancerous if they are not all removed.  

Molar pregnancies are rare but they are the most common type of GTD. In the UK, about 1 in 590 pregnancies is a molar pregnancy. In Asian women, molar pregnancies are about twice as common as in non-Asian women.

Molar pregnancies can be complete or partial.

Complete molar pregnancy

A complete molar pregnancy happens when a sperm fertilises an empty egg that contains no genes from the woman. 

In a complete molar pregnancy, no parts of a baby (foetal tissue) form. There is only molar tissue in the womb. 

You have surgery or drug treatment to remove the molar tissue. Afterwards, in around 15 out of 100 women (around 15%) some molar tissue remains in the deeper tissues of the womb or other parts of the body. This is called an invasive mole or persistent trophoblastic disease (PTD). An invasive mole is usually treated with chemotherapy. 

Partial molar pregnancy

A partial molar pregnancy forms when two sperm fertilise the egg at the same time. So there is one set of female chromosomes and two sets of male chromosomes..

There may be some foetal tissue within the molar tissue. So it may look like there is a foetus on an ultrasound scan. But sadly, the foetal tissue cannot develop into a baby.

You have surgery or drug treatment to remove the molar tissue. Most women with a partial molar pregnancy don't need to have any further treatment. But around 1 in 100 women (around 1%) have some remaining abnormal cells in the deeper tissues of the womb or other parts of the body. This is called an invasive mole or persistent trophoblastic disease (PTD). An invasive mole is usually treated with chemotherapy. 

Twin pregnancy

Very rarely a twin pregnancy will show a normal developing baby and a molar pregnancy at the same time.  

For many women, it is possible for the pregnancy to continue. But if you have complications you may not be able to go ahead with the pregnancy. This is obviously a very difficult situation. Your doctor will tell you about the risks involved. They can refer you to one of the national molar pregnancy treatment centres for advice.

You might also have counselling so that you can think through any choices you might have before you make a decision.

Invasive mole

Usually any molar cells left after treatment will die off by themselves. But sometimes these cells will continue to grow and form a tumour. This is called an invasive mole or persistent trophoblastic disease (PTD). 

If it is not treated it can grow quickly and cause a number of medical problems. Most women with an invasive mole need to have chemotherapy treatment.