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Hydatidiform mole

Page last reviewed: 13/07/2011

A molar pregnancy is a pregnancy that has not been successful, and means that a baby will not develop.

What happens

In a normal pregnancy, the placenta provides nourishment to a 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 rapidly growing within the womb instead of a normally developing baby. This mass of cells is referred to as a "mole".

Other terms for a molar pregnancy are gestational trophoblastic tumour, trophoblastic disease and hydatidiform mole.

Removing the mole

A mole is classed as a benign growth, which means it is not cancerous. However, it must be removed surgically. This is done under the care of a gynaecologist.

The operation to remove the mole is called dilatation and curettage, which involves removing the lining of the womb.

How common is it?

It is uncommon: about one in every 700-800 pregnant women will develop a molar pregnancy.

Symptoms and diagnosis

Some women will develop abnormal symptoms of pregnancy. The most likely symptom of a molar pregnancy is vaginal bleeding. Your doctor will usually be able to identify a molar pregnancy during the first ultrasound scan, at 10-16 weeks of pregnancy.

A partial vs a complete mole

A complete molar pregnancy is a mass of rapidly growing cells that does not contain a foetus (unborn baby). It develops when there is only genetic material from the father, and the mother's genetic material is missing.

A partial mole occurs when there is genetic material from both the father and mother, but there are two sets of material from the father. A foetus may be visible on an early ultrasound, but this is abnormal and will not survive beyond the first three months of pregnancy.

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Dilation and curettage


Following the operation to remove the mole, some cells may be left in the womb. These cells usually die off over time.

To check that this has happened, all women must undergo routine monitoring of the hormone hCG (human chorionic gonadotrophin) . HCG is a pregnancy hormone produced by the mole cells. It can be detected in blood and urine tests. hCG monitoring will identify the small number of women who develop a persistent or invasive mole. There is a 10% chance of this happening, and if it does happen it is classed as malignant (cancerous).

The risk of malignant change of a partial mole pregnancy is 1%. The risk of malignant change of a complete mole is 15%.Invasive molar pregnancy is usually treated with chemotherapy.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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