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Molar pregnancy

Hydatidiform Mole

A molar pregnancy (hydatidiform mole) is when a mass of tissue grows inside your womb (uterus) that will not develop into a baby. It is the result of abnormal conception. It may cause bleeding or severe vomiting in early pregnancy and is usually picked up in an early pregnancy ultrasound scan. It needs to be removed and most women can expect a full recovery.

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What is a molar pregnancy?

Is a molar pregnancy a tumor?

A molar pregnancy is a growing mass of tissue inside the womb (uterus) that will not develop into a baby. It happens due to a problem with conception, when an egg becomes fertilised by sperm.

Types of molar pregnancy

There are two types of molar pregnancy: a complete molar pregnancy or partial.

Complete molar pregnancy

A complete molar pregnancy happens when an egg with missing DNA is fertilised by one or two sperm cells. So, the fertilised cell only contains chromosomes from the father, and none from the mother.

In a complete molar pregnancy, there is no fetus. Instead, abnormal placenta grows and forms fluid-filled cysts.

Partial molar pregnancy

A partial molar pregnancy happens when a normal egg is fertilised by two sperm, or one sperm that duplicates itself. This means that the fertilised egg has one set of chromosomes and two from the father.

In a partial molar pregnancy, the fetus may start to grow, but it can't survive the pregnancy. Abnormal placenta cells also grow and can form fluid-filled cysts. It often ends in miscarriage.

Partial and complete hydatidiform moles will not produce a live baby. (They are 'non-viable' pregnancies.) A pregnancy that results in a hydatidiform mole is called a molar pregnancy.

A molar pregnancy is part of a group of disorders called gestational trophoblastic disease (GTD). The trophoblast is the outer layer of cells around an embryo, which normally grows into the placenta and the membranes surrounding the fetus. Partial and complete hydatidiform moles are benign (not cancerous) forms of GTD. However, there is a risk that a hydatidiform mole can develop into a cancerous (malignant) type of GTD, known as gestational trophoblastic neoplasia (GTN). This is described below.

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What are the symptoms of a molar pregnancy?

Women with a hydatidiform mole usually have higher than average levels of the pregnancy hormone human chorionic gonadotrophin (hCG) compared with women with a normal pregnancy.

This hormone is produced by the trophoblastic tissue. It is the hormone that is detected in a standard pregnancy test. The high levels of hCG occur because there is an excessive amount of trophoblastic tissue with a molar pregnancy. The high hCG levels are responsible for some of the symptoms.

Pregnancy symptoms

You may have signs of pregnancy, including:

  • Your periods stopping.

  • Feeling sick (nausea).

  • Being sick (vomiting).

  • Breast tenderness.

You may notice that you feel bigger than expected for the number of weeks you are pregnant. This is because a molar pregnancy grows more quickly than a normal pregnancy would, due to the abnormally developing trophoblastic tissue.

Vaginal bleeding

You may have vaginal bleeding or dark discharge early in the pregnancy. This is one of the most common symptoms of molar pregnancy. Many women suspect that they are having a miscarriage.

No symptoms

Some women with a hydatidiform mole have no symptoms. The molar pregnancy is diagnosed after a routine pregnancy ultrasound scan (see below).

Rare symptoms

In very rare cases, you may have problems with high blood pressure, very bad nausea and vomiting (hyperemesis gravidarum) or symptoms of an overactive thyroid gland may develop.

What causes a molar pregnancy?

Molar pregnancies are caused by problems at the point of conception - when an egg is fertilised by a sperm cell.

Complete molar pregnancies happen when an 'empty' egg (with no maternal DNA) is fertilised by one or two sperm cells. Normally, these eggs would die, but in a complete molar pregnancy, they survive and implant in the uterus.

Incomplete molar pregnancies happen when a normal egg is fertilised by two sperm cells, or one sperm cell that duplicates its DNA - meaning there are too many sets of chromosomes in the fertilised egg.

We don't know exactly what causes the conception problems in molar pregnancies to happen. It's not due to anything that either parent has done wrong.

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How rare is a molar pregnancy?

Hydatidiform mole is rare in the UK. There is about 1 molar pregnancy for every 714 live births. This works out to be fewer than 1,000 hydatidiform moles diagnosed per year in the UK.

Any woman of childbearing age can develop a molar pregnancy but women who are aged under 15 and over 45 have a higher risk.

Women who have had a molar pregnancy are more likely to have a second one, although the risk is still low; about 1 in 80 women who have had a first molar pregnancy develop a second one. The risk increases with two molar pregnancies; after two molar pregnancies, the risk of having a third molar pregnancy is about 1 in 4.

Risk factors include women of Asian backgrounds, who for some reason are more likely to have a molar pregnancy compared with women from non-Asian backgrounds.

How is a molar pregnancy diagnosed?

If you develop bleeding in early pregnancy, your doctor will usually arrange an ultrasound scan. Sometimes, the picture on the ultrasound scan can be quite characteristic if you have a hydatidiform mole. However, at other times the ultrasound picture may just look like a miscarriage.

If your doctor suspects that you have a hydatidiform mole, they may also suggest a blood test to check your levels of hCG. Higher than normal levels can help with the diagnosis.

If you have an ultrasound picture that looks like a miscarriage and higher than normal hCG levels, the definite diagnosis of hydatidiform mole is made when the pregnancy is removed and examined under a microscope (see below).

If you do not have bleeding or other symptoms, the diagnosis of hydatidiform mole is usually suspected when you have a routine pregnancy ultrasound scan. The further through the pregnancy you are, the more characteristic the ultrasound picture of a molar pregnancy becomes.

What is the treatment for a molar pregnancy?

If you have a hydatidiform mole, you will need to have it removed. This means having a small operation. This is done in hospital by a doctor who is a gynaecology specialist. You will be given an anaesthetic.

In most cases, a small tube is passed into your womb (uterus) through the opening of your uterus (your cervix) and the abnormal tissue is removed by suction. The tissue is then sent off to the laboratory for examination under the microscope.

Follow-up care

As mentioned above, there is a small risk that you can develop GTN - the cancerous form of the condition - after a molar pregnancy. There is a risk that this can spread (metastasise) to other parts of your body including your lung, liver or brain. Therefore, you need close follow-up after a molar pregnancy to monitor for this.

When you are first diagnosed with a hydatidiform mole, your hCG level will be raised. When the hydatidiform mole is treated (removed), the hCG level will usually return to a normal, non-pregnant amount and should remain so.

If you develop GTN, the hCG level can remain elevated or continue to rise further. So, this blood test is a good way to check for the possible development of GTN.

A molar pregnancy is relatively rare in the UK. Therefore, if you are diagnosed as having a hydatidiform mole, it is very important to be registered with one of the three specialist centres (located in either Dundee, Sheffield or London).

Your gynaecologist should arrange this for you. These centres will arrange follow-up blood/urine tests for hCG to look for any evidence of GTN developing. They will advise you on how often to have the tests. The tests should be arranged by the specialist centres, not your GP. Follow-up will be for at least six months after your hCG levels have returned to normal provided you do not develop GTN.

Is treatment possible if I develop gestational trophoblastic neoplasia?

Yes. Treatment is possible and is effective in almost all cases.

About 1 in 200 women with partial molar pregnancies and 15 in 100 women with complete molar pregnancies will need treatment because hCG levels are not returning to normal. The exact treatment given depends on things such as your hCG levels and whether there is spread to other parts of the body.

Treatment involves injections of different medications (chemotherapy) such as methotrexate, etoposide and dactinomycin (actinomycin D). Treatment is continued until your hCG level has returned to normal and then for some weeks afterwards.

When is it safe to try for another baby?

Your specialist will advise you when it is safe to try for another baby if you have had a molar pregnancy or GTN. Usually you will be advised not to become pregnant until your specialist follow-up is complete, and for a year after finishing chemotherapy.

You can use most types of contraception without having to wait for your tests to return to normal. This includes hormone contraception, such as the combined oral contraceptive pill or progestogen-only contraceptive pill, if there are no factors which make them unsuitable for you personally.

It is then very unlikely that you would have another hydatidiform mole if you became pregnant again. Having had one hydatidiform mole makes it more likely that you can have another but it is still very unusual. Fewer than 2 in 100 women who fall pregnant after previously having a molar pregnancy will develop another molar pregnancy.

Women who have been treated for GTN and then go on to have a successful pregnancy may need to have their hCG monitored after pregnancy - this is not necessary for those with GTD which does not develop into cancer.

What is the outlook (prognosis)?

Most women who have a molar pregnancy have an excellent outlook. As mentioned above, about 1 in 200 women with partial molar pregnancies and 15 in 100 women with complete molar pregnancies will need treatment for GTN.

In those who develop GTN, the cure rate is very close to 100% and around 80% will become pregnant in the future. There is however an increased risk of premature menopause after the use of some chemotherapies; this should be discussed at the time of treatment, particularly for older women.

How does conception work?

Chromosomes are found in the centre (the nucleus) of the cells in your body. They carry genetic information in the form of genes. Most of the cells in your body contain 46 sets of chromosomes arranged in 23 pairs.

One chromosome from each pair is inherited from your mother and the other is inherited from your father. These 46 chromosomes make up your genetic code. This code determines what you look like, how your body functions, whether you are male or female and sometimes even whether you will develop certain diseases.

However, the reproductive cells - eggs (ova) in women and sperm in men - only have 23 chromosomes. This is so that, when a sperm fertilises an egg during normal conception, the child that is produced has 46 chromosomes in each cell - 23 from their mother and 23 from their father. The fertilised egg (ovum) has a complete set of genetic material.

After fertilisation, some cells from the fertilised ovum (called the trophoblast cells) develop into the placenta and membranes that form around the developing baby. The placenta provides oxygen and nutrients to the developing baby. The other cells develop into the embryo from which the baby grows.

As the placenta develops, the trophoblast cells grow into and attach to the lining of the womb (uterus), allowing the pregnancy to implant in the uterus.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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