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What is a molar pregnancy?

Men and women discussing gestational trophoblastic tumours

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What is a molar pregnancy?

Molar pregnancy is a type of gestational trophoblastic tumour (GTT). It happens when the normal fertilisation of an egg goes wrong. This leads to the growth of abnormal cells or clusters of fluid filled sacs inside the womb. In the UK, about 1 in 590 pregnancies are a molar pregnancy. Most are not cancerous (they are benign). Even those that can spread beyond the womb are curable. 

Types of molar pregnancy

If you have a complete mole no parts of a baby (foetal tissue) form. The molar tissue is removed with surgery. 10 to 15 out of 100 women who have a complete mole later develop a cancerous gestational tumour (choriocarcinoma). These tumours can be treated and cured with chemotherapy.

If you have a partial mole there may be some foetal tissue in the womb, alongside the molar tissue. It is important to understand that the foetal tissue cannot develop into a baby and it is removed with surgery. 1 out of 100 women who have a partial mole later develop a choriocarcinoma and will need chemotherapy.

Very rarely a twin pregnancy will show a developing baby and a molar pregnancy. This is a very difficult situation. You will need to talk to your doctor about the risks and the choices available to you.

Persistent trophoblastic disease is when you have had treatment to remove a molar pregnancy but still have some abnormal cells left.

 

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About 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 one of a group of conditions known as gestational trophoblastic tumours (GTTs). Molar pregnancies used to be called hydatidiform mole but now most people call them molar pregnancies. Most molar pregnancies are benign (not cancerous). They can spread beyond the womb in some women, but are still curable.

Molar pregnancies are rare but they are the most common type of gestational trophoblastic tumour. In the UK, about 1 in 590 pregnancies is a molar pregnancy. In Asian women, molar pregnancies are about twice as common as in caucasian women.

Molar pregnancies can either be complete or partial.

 

Complete molar pregnancy

If you have a complete mole, no parts of a baby (foetal tissue) are formed. There is only molar tissue in the womb. The molar tissue is usually removed with surgery. Afterwards, in around 10 to 15 out of 100 women some molar tissue remains in the deeper tissues of the womb or other parts of the body. This is called a persistent gestational tumour. These women need to have chemotherapy, which can completely get rid of the abnormal cells.

 

Partial molar pregnancy

If you have a partial mole there may be some foetal tissue in the womb, alongside the molar tissue. It is important to understand that the foetal tissue cannot develop into a baby, although on an ultrasound scan it may look like a foetus. The molar tissue needs to be surgically removed. 

Most women with a partial mole do not then need to have any further treatment. But around 1 in 100 women have some remaining abnormal cells in the deeper tissues of the womb or other parts of the body. This is called a persistent gestational tumour. These women need chemotherapy treatment, which can completely get rid of the abnormal cells.

 

Twin pregnancy

Very rarely a twin pregnancy will show a normal developing baby and a molar pregnancy at the same time. This happens in fewer than 1 in 100 cases of GTT (less than 1%). The risk of developing persistent trophoblastic disease is higher in this situation. For many women it is possible for the pregnancy to continue. Reports show that in about 25 to 40 out of every 100 of these cases (25 to 40%) the healthy baby can be delivered and survives. 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 may refer you to one of the national molar pregnancy treatment centres for advice. You may also have counselling so that you can think through any choices you might have before you make a decision.

 

Persistent trophoblastic disease

Persistent trophoblastic disease is when you have had treatment to remove a molar pregnancy but still have some molar tissue left and it grows and forms a new tumour. This occurs in about 1 in 12 women (8%) after a molar pregnancy. If it is not treated it can grow quickly and cause a number of medical problems. Most women with persistent trophoblastic disease need to have chemotherapy treatment. There is information about persistent trophoblastic disease and its treatment in this section.

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Updated: 29 April 2014