What is a molar pregnancy and are you at risk?

Dr Juliet McGrattan (MBChB)
·7-min read
Photo credit: Westend61 - Getty Images
Photo credit: Westend61 - Getty Images

From Netdoctor

Recently diagnosed with a molar pregnancy or worried you might be experiencing one? Molar pregnancies are rare and only happen in about one in 700 pregnancies. All the same, it can be a real shock to be given the diagnosis of a molar pregnancy so it is perfectly normal if you feel very upset.

It's important to know that molar pregnancies are not caused by anything you did or didn’t do and support is available. Dr Juliet McGrattan is here to simplify a confusing topic and offer reassurance and guidance:

What is a molar pregnancy?

Molar pregnancies are not normal pregnancies, they cannot survive and they never result in the birth of a baby. Things go wrong when the sperm fertilises the egg and although the egg implants into the wall of the uterus as it would in a normal pregnancy, the contents of the egg do not lead to a normal embryo developing.

Molar pregnancies are a type of trophoblastic disease. Trophoblasts are the cells which become the placenta in a normal pregnancy. In a molar pregnancy, trophoblast cells rapidly multiply and grow out of control.

The full name for a molar pregnancy is a hydatidiform mole or hydatid mole which means a mass of fluid-filled cells or cysts. When seen on a scan, its appearance is often compared to a bunch of grapes.

Photo credit: VioletaStoimenova - Getty Images
Photo credit: VioletaStoimenova - Getty Images

Types of molar pregnancy

There are two main types of molar pregnancy:

  1. Complete moles: these occur when a sperm (or sometimes two sperm) fertilise an empty egg. The genetic material is all from the sperm. No foetal tissue can develop.

  2. Partial moles: two sperm fertilise one egg. Because there is male and female genetic material, there is usually evidence of a foetus or of foetal red blood cells.

In both complete and partial moles there is an overgrowth of the trophoblastic cells.

Molar pregnancy symptoms

You may not be aware you have a molar pregnancy. In fact this is the most common scenario. This means it can come as quite a shock.

Some women attend for their first pregnancy scan around 12 weeks of pregnancy and discover that rather than showing a normally developing foetus, there is just a mass of cells classic of a molar pregnancy.

Similarly, women who have had a miscarriage that was treated in hospital may be contacted to say that the tissue that was removed from their womb has shown evidence of a molar pregnancy when it was looked at under the microscope.

Sometimes before you have your routine scan there are some symptoms that could potentially indicate a molar pregnancy. If you have a positive pregnancy test and are experiencing any of the following you should speak to your doctor:

• Irregular vaginal bleeding

The blood may look darker than normal and you might see some tiny grape-like cysts in the blood.

• Hyperemesis

Hyperemesis means excessive vomiting and nausea. Severe morning sickness is unusual in very early pregnancy so this can be a symptom of a molar pregnancy.

• Excessive uterine enlargement

If your bump seems much bigger than it should for the stage of pregnancy you are at then it could be a sign of rapidly multiplying molar pregnancy cells.

• Persistent bleeding after pregnancy

Persistent vaginal bleeding after you have been pregnant, for example following a miscarriage or termination of pregnancy. You would normally expect this to have settled within eight weeks.

Photo credit: valentinrussanov - Getty Images
Photo credit: valentinrussanov - Getty Images

What causes a molar pregnancy?

It’s not known why molar pregnancies occur, they just develop on their own. If you have had a molar pregnancy you can be certain that it is not your fault and there was nothing you could have done to prevent it.

Molar pregnancies are rare – they occur in around one in 700 pregnancies in the UK. A consultant in Obstetrics and Gynaecology would expect to see one new case every two years.

Although we don’t know why molar pregnancies happen and they can’t be prevented, there do seem to be some women who have a higher risk of a molar pregnancy than other women. These are:

  • Young teenagers

  • Women over 45

  • Women from an Asian background

  • Women who have had a previous molar pregnancy

How are molar pregnancies diagnosed?

If your doctor suspects a molar pregnancy, then he or she will either arrange some tests for you or refer you directly to an early pregnancy assessment unit or a gynaecologist.

• Blood test

You will have a blood test to look at the level of hCG (human chorionic gonadotropin). This is the pregnancy hormone that is detected in urine in home pregnancy tests. The level of hCG gradually increases during pregnancy. It is made by the trophoblastic cells. In a molar pregnancy the trophoblastic cells are rapidly multiplying so the level of hCG increases quickly and is usually much higher than would be expected in a normal pregnancy. It’s the high levels of hCG that can make you feel unwell with nausea and vomiting.

• Ultrasound scan

An ultrasound scan will give a clear picture of the size and characteristics of pregnancy to determine whether they fit with a molar pregnancy.

• Histology

To give a definite diagnosis of a molar pregnancy the tissues need to be examined under a microscope (histology). They will show the characteristic changes which confirm the diagnosis.

Molar pregnancy treatment

You may not need any treatment because molar pregnancies often end in miscarriage and the tissues come away naturally in vaginal bleeding.

If this has not happened, then they can be surgically removed. This procedure is done under anaesthetic and the pregnancy tissues extracted through the vagina.

After treatment the doctor will check blood and urine tests for several weeks to make sure that your hCG levels have returned to normal. This is a good indication that all the molar tissue has been cleared.

Sometimes a medication is required, a form of chemotherapy, to clear molar pregnancy tissue. Only around one in 10 women with a molar pregnancy will require this.

Other types of molar pregnancy

The following different types of molar pregnancies can also occur:

• Invasive moles

When a molar pregnancy has implanted in the lining of the uterus and the trophoblastic cells are multiplying, they can sometimes start to burrow deeply into the wall. This is called an invasive mole. Invasive moles are rare but they are more serious. They are harder to remove and the cells can become cancerous and spread to other parts of the body. It is more common for complete moles to become invasive moles than it is for partial moles. Thankfully treatment for invasive moles has a cure rate close to 100 per cent.

• Twin pregnancies

Very rarely a molar pregnancy develops in a twin pregnancy. One foetus develops normally but the other is molar. It is possible for the normal pregnancy to continue but there is a higher risk of miscarriage or premature birth for the twin as well as increased risk of pre-eclampsia (high blood pressure in pregnancy) for the mother. Treatment in a specialist centre is required.

• Ectopic pregnancies

It is also possible for a molar pregnancy to be an ectopic pregnancy. Ectopic pregnancies develop outside of the uterus, usually in the fallopian tubes. Lower abdominal pain with or without vaginal bleeding are signs of an ectopic pregnancy. Molar ectopics are treated in the same way as normal ectopics.

Molar pregnancy treatment follow up

Treatment for molar pregnancies is usually done in a specialist centre and a register of women who have had a molar pregnancy is kept.

Following a complete mole, women will be followed up for six months after their hCG levels have returned to normal.

Partial moles are only followed up for 4 weeks after a normal hCG level because complications are rarer.

Getting pregnant after a molar pregnancy

It is important that your treatment and follow up after a molar pregnancy are complete before you try for another pregnancy. You should use contraception to prevent pregnancy happening before this time.

If you had chemotherapy treatment for your molar pregnancy, you need to wait 12 months after it has finished before you try to get pregnant again.

While molar pregnancies are more common in women who have already had one (a risk of around one in 100), it is reassuring to know that molar pregnancies don’t affect your fertility and 80 per cent of women go on to have a normal pregnancy.

Further help and support

For further help and support with anything related to molar pregnancies or miscarriage try one of the following resources:

  • Samaritans: free, 24/7 emotional support to anyone in distress.

  • Tommy's: UK charity carrying out research into pregnancy loss.

  • Miscarriage Association: support for anyone affected by miscarriage, ectopic or molar pregnancy.

  • Molar pregnancy: community website offering support and information for women who have experienced a molar pregnancy.

Last updated: 18-02-2021

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