17 May 2023

Molar Pregnancy: definition, causes, symptoms

Molar pregnancy
Medical Director
Cesar Diaz-Garcia
MD PhD Assoc Prof
IVI London


Molar pregnancy, also known as a hydatidiform mole, is a relatively rare condition in which fertilisation takes place, but the resulting embryo has an incorrect or imbalanced number of cells in its genetic material and has no chance of developing into a baby. A molar pregnancy can happen completely by chance and can be an extremely distressing experience. Couples should not blame themselves or blame their actions: it is simply one of the sad and unexplained occurrences which can happen in the complicated world of human reproduction.

The distress caused by such an event can be exacerbated by the fact that so little is known about a molar pregnancy, and many have not even heard of it.

What is a molar pregnancy?

Types of molar pregnancy

A molar pregnancy happens when an abnormally fertilised egg implants in the womb. The cells that would normally develop into the placenta grow too quickly, occupying the space normally taken up by the developing embryo. Its name comes from the Latin mole, meaning a mass, in this case, a mass of cells.

There are two types of molar pregnancy, with similar symptoms and possible complications:

  • A partial mole results from two sperm, instead of one, fertilising an egg at the same time. The resulting pre-embryo has too much genetic material to be able to develop in a normal way.
  • A complete mole results from an ‘empty’ egg, containing no genetic material, being fertilised by a sperm cell. The result is similar to a partial mole, with there being an incorrect number of chromosomes to allow for an embryo to develop.

What are the symptoms of a molar pregnancy?

Symptoms of molar pregnancy are often difficult to detect. During a molar pregnancy, the body produces a very high amount of hCG, the pregnancy hormone, which makes its symptoms hard to distinguish from the symptoms of a normal pregnancy. However, there are notable symptoms which if you notice, you should contact your GP or midwife for further investigations:

  • Vaginal bleeding (particularly during early pregnancy)
  • Severe morning sickness
  • An unusually large bump for an early stage of pregnancy

Very rarely, complications can arise in a condition known as gestational trophoblastic neoplasia (GTN). This can present in:

  • Invasive moles (groups of cells that can spread in the muscle layer of the uterus)
  • Choriocarcinomas (malignant tumours that grow in the muscle layer of the uterus)
  • Trophoblastic tumours, such as placental-site trophoblastic tumours (PSTT) and epithelioid trophoblastic tumours (ETT)

While most hydatidiform moles are benign and not dangerous, GTN is cancerous. However, when treated, it has an almost 100% cure rate. 

How is a molar pregnancy diagnosed and treated?

A molar pregnancy can be detected during an ultrasound scan, so it is common for diagnosis to happen during the first ultrasound scan at around 8 to 14 weeks. However, a molar pregnancy is prone to spontaneously miscarry, so many learn they have the condition before beginning their early pregnancy care.

Sadly, a molar pregnancy is not viable and will not be able to survive. If it does not end naturally with a spontaneous miscarriage, removing the pregnancy with medication is possible. Surgery may be required in some cases. If you need surgery, you would be given general anaesthetic to limit any physical discomfort.

If you live in the UK and experience a molar pregnancy, you will be asked to register with a specialist centre so that experts in the field can provide your care. These specialist centres are located in hospitals in London, Sheffield and Dundee. Follow-up care after treatment will last for around six months and includes blood and urine tests carried out through your GP surgery. These tests will monitor your human chorionic gonadotropin (hCG) levels and check when they have returned to normal.

Who is most at risk of molar pregnancy?

It is important to remember that molar pregnancies are not a result of anything you or your partner did, or didn’t do. Although there is no preventative action that you can take to avoid a molar pregnancy, doctors have noted that there are a few factors that can lead to you being at a higher risk:

  • Age: Teenage girls and women over 45 have a higher risk of having a molar pregnancy than women in the middle of their childbearing years.
  • Ethnicity: Women of Asian origin have twice the average risk of molar pregnancy.
  • Previous molar pregnancy: For women who have never experienced a molar pregnancy, the chance of having one is around 1 in 600; if you have had a molar pregnancy previously, the risk increases to 1 in 80.
  • Nutrition: Women with low levels of folic acid and carotene (a form of Vitamin A) have been known to be at a slightly higher risk of molar pregnancy. However, it’s important to note that women trying to get pregnant should not take Vitamin A supplements as these can be harmful to a foetus. The best protection is to ensure a good vitamin intake from a balanced, healthy diet.

Can a molar pregnancy impact your chances of having a healthy pregnancy in the future?

Having a molar pregnancy has no impact on your ability to get pregnant in the future. It also has no impact on your ability to have a healthy baby. However, it’s important that you use contraception during the period of follow-up care. You should also wait for your doctor’s go-ahead before trying for a baby: this will happen once your hCG levels have returned to normal.

What help is available?

It can take time to recover from a molar pregnancy, both physically and emotionally. Your GP or midwife can advise on what help is available and may be able to point you towards support services in your area. You can also find information, support and counselling services through these UK organisations:

Getting in touch with IVI

After molar pregnancy, a full recovery can easily be made. You shouldn’t have any fears about your chances of having a healthy pregnancy in the future. However, a molar pregnancy can be an extremely challenging experience and can prompt anxiety and concerns for future pregnancies. If you would like to speak to someone or have any questions, IVI is always here to help. Just contact us and we’ll take it from there.

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