A molar pregnancy is an unsuccessful pregnancy, where the placenta and fetus do not form properly and a baby does not develop.
In 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 developing into a baby. The abnormal cells are referred to as a ‘’mole’’, which is from the Latin for mass or lump.
Molar pregnancy is also called a hydatidiform mole and is a pre-cancerous form of gestational trophoblastic disease.
Molar pregnancies are caused by an imbalance in genetic material (chromosomes) in the pregnancy. This usually occurs when an egg that contains no genetic information is fertilised by a sperm (a complete molar pregnancy), or when a normal egg is fertilised by two sperm (a partial molar pregnancy).
In complete molar pregnancy, the embryo does not develop at all. In a partial molar pregnancy, a fetus can develop but never results in a viable baby, due to the imbalance between the male and female chromosomes.
A molar pregnancy is not caused by anything that you or your partner does or does not do.
How common is molar pregnancy?
Molar pregnancies are rare. About one to three in every 1,000 pregnancies turn out to be molar.
Factors that increase the risk of molar pregnancies are thought to include the following:
- Age – complete molar pregnancies are more common in teenage women and women over 45 years old. The age has little or no effect on the risk of partial molar pregnancies.
- Previous molar pregnancy – if you have had one molar pregnancy previously, your chance of having another one is around one to two in 100, compared with one in 600 for women who have not experienced a molar pregnancy. If you have had two or more molar pregnancies, your risk of having another is around 15-20 in 100.
- Ethnicity – molar pregnancies are more common in Asian countries, such as Taiwan, the Philippines and Japan, and also among Native Americans. However, in recent years, the differences in the incidence of molar pregnancy between these communities and the general population have become less marked.
There are often no signs that a pregnancy is a molar pregnancy. In most cases, the problem is first spotted during an ultrasound scan, which may be the first pregnancy scan at 10-14 weeks.
If there are symptoms, they usually appear between 4 and 12 weeks of pregnancy. The most common symptom is bleeding or losing brown-red fluid from the vagina.
Sickness and vomiting may be more severe than in a normal pregnancy.
Bleeding usually requires an ultrasound scan. If this scan is abnormal, an evacuation of the uterus is performed. This is when the molar pregnancy is removed, usually with a surgical procedure called suction evacuation. The surgery involves opening your cervix (neck of the womb) and removing any remaining tissue with a suction device. Tissue from the pregnancy is sent to a laboratory to confirm whether it is a molar pregnancy.
If a woman has a miscarriage or a termination for other reasons, tissue may be sent to a laboratory for analysis. This may confirm that the pregnancy was molar, even if a molar pregnancy was not suspected.
A molar pregnancy usually needs to be removed surgically. This is performed with a suction evacuation, under the care of a gynaecologist.
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 removal of the molar pregnancy, some cells will be left in the womb. These cells usually die off over time in around 90% of women.
To check the cells have died, all women having experienced a molar pregnancy in the UK will undergo monitoring of the hormone hCG (human Chorionic Gonadotrophin) via the National Trophoblastic Screening Centre’s surveillance programme. hCG is the pregnancy test hormone produced by a normal placenta, but also by the mole cells, and is the hormone detected in a pregnancy test. It can also be detected in blood and urine tests.
Women on the surveillance programme send in blood or urine samples every two weeks. This is so they can be monitored for signs of persistent trophoblastic disease, which is a risk after a molar pregnancy.
Persistent trophoblastic disease needs further treatment with chemotherapy.
It is recommended that you do not become pregnant until you complete your hCG hormone monitoring, following a molar pregnancy. This normally happens within a few months, but in some cases can take up to a year.
Most women who have received chemotherapy treatment for persistent trophoblastic disease will have started their periods again after six months post treatment.
The Charing Cross Hospital trophoblastic disease centre suggest avoiding pregnancy for 12 months after finishing chemotherapy.
While physical recovery is often relatively quick, it can take longer to recover emotionally from a molar pregnancy.
Not only does a molar pregnancy involve the loss of a fetus, it also carries the slight risk of a cancerous growth. This can cause an enormous emotional strain.
Communicating your feelings with your partner, a doctor, a counsellor, or someone else who has gone through a similar experience can be beneficial.
There are support groups and forums that can help people handle the stress of a molar pregnancy: