A molar pregnancy is an unsuccessful pregnancy where the placenta and foetus do not form properly and a baby does not develop.
In a normal pregnancy, the placenta provides nourishment to the 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 that can grow as fluid-filled sacs (cysts) with the appearance of white grapes. These cells grow rapidly within the womb instead of a normally developing baby. The abnormal cells are referred to as a 'mole', which is from the Latin for mass or lump.
Other terms for a molar pregnancy are gestational trophoblastic tumour, trophoblastic disease and hydatidiform mole.
Molar pregnancies are caused by an imbalance in genetic material (chromosomes) in the pregnancy. Most often, this occurs when an egg that contains no genetic information is fertilised by a sperm, or when a normal egg is fertilised by two sperm.
How common is molar pregnancy?
Molar pregnancies are rare, only about one in every 600-800 pregnant women in the UK will develop a molar pregnancy. This amounts to less than 1,500 a year.
Factors that increase the risk of molar pregnancies are thought to include:
- Age – Molar pregnancies, particularly partial molars (see below), are more common in teenage pregnancies or women over the age of 45.
- Previous molar pregnancy – If you have had a molar pregnancy before, then the chance of another one is around 10 times higher than if you haven’t.
Types of molar pregnancy
There are two main types of molar pregnancy, depending on the balance of chromosomes in the egg. These are:
- complete moles, when no normal placental tissue forms and no foetus develops, instead a mass of abnormal cells grow
- partial moles, when some normal placental tissue forms along with an abnormal foetus, but the foetus does not survive longer than three months
In very rare cases, a twin pregnancy can include a normal foetus and a mole.
Symptoms and diagnosis
In many cases, there are no signs that a pregnancy is a molar pregnancy, although you may get bigger more quickly than usual and suffer with bad morning sickness.
If there are symptoms, the most common is dark-coloured vaginal bleeding. This usually starts to occur about six-12 weeks after conception.
In most cases, the problem is first spotted during an early ultrasound scan that usually takes place between weeks 10-16 of pregnancy.
Although a mole is classed as a benign growth, which means it is not cancerous, 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 (D&C), which involves removing the contents of the womb.
In some cases, molar pregnancy can be treated with the removal of the womb (hysterectomy), but this is usually only if you no longer wish to have children.
Almost all cases of molar pregnancy are successfully cured.
Following the operation to remove the mole, some cells will always be left in the womb. These cells usually die off over time in around 90% of women.
To check this has happened, all women must undergo routine monitoring of the hormone hCG (human chorionic gonadotrophin) via the National Trophoblastic Screening Centre. hCG is the pregnancy test hormone produced by the mole cells. It can be detected in blood and urine tests.
Hormone monitoring will also identify the small number of women who develop a persistent or invasive mole (see below).
It is recommended that you do not get pregnant again until your hormone levels return to normal, which normally happens within two months, but can take up to a year. The risk of reocurrence, while higher than the general population, is still relatively small. The chances of a molar pregnancy happening again is only about 1% and most women go on to have successful pregnancies.
In rare cases, a persistent or invasive mole can develop. 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.
In all cases, malignant moles are successfully treated, usually with only gentle chemotherapy being required.