What causes a miscarriage?

We don’t know why all miscarriages happen, but sometimes there can be underlying reasons for this type of pregnancy loss. It’s very unlikely that a miscarriage was caused by something you did or didn’t do.

At the moment, it sadly isn’t always possible to give a reason why a woman has a miscarriage. That’s why Tommy’s opened the UK’s only miscarriage research centre dedicated to understanding miscarriage and preventing it.

However, there are some reasons why a miscarriage may happen that we do know about.

Early miscarriage

If a miscarriage happens during the first 3 months of pregnancy (known as early miscarriage), it’s usually caused by chromosomal abnormalities in the baby. These happen by chance.Chromosomes are blocks of DNA, which contain instructions for your baby’s development.

Sometimes something can go wrong at the point when you get pregnant and the baby gets too many or not enough chromosomes. If this happens, the baby can’t develop properly.

If there's a problem with the development of the placenta, this can also lead to a miscarriage. The placenta is an organ that helps your baby grow and develop. It’s attached to the lining of the womb and is connected to your baby by the umbilical cord.

Late and recurrent miscarriage

There are several factors that may play a part in causing recurrent and late miscarriage. Recurrent miscarriage is usually defined as the loss of 3 or more consecutive pregnancies. Late miscarriage is when a baby dies between 14 and 24 weeks of pregnancy.

Antiphospholipid syndrome (APS)

Antiphospholipid syndrome (APS) is a syndrome that makes your blood more likely to clot. This is not common but can cause recurrent miscarriage and late miscarriage.

Thrombophilia

Thrombophilia is an inherited condition that means that your blood may be more likely to clot. This may cause recurrent miscarriage and late miscarriages.

Genetic factors

Recurrent miscarriage is sometimes caused by genetic factors. If one partner has an abnormality in one of their chromosomes, it can sometimes cause repeated miscarriages. The partner may not be aware of this abnormality. This happens in around 2–5% of cases.

Find out more about conception and your baby’s genes.

Weak cervix

If you miscarry between 14 to 23 weeks of pregnancy, it may be due to a weak cervix. Unfortunately, this can be difficult to diagnose when you are not pregnant. But it may be suspected if your waters broke early in a previous pregnancy, or if the neck of the womb opened without any pain in a previous miscarriage.

Find out more about cervical weakness.

Developmental problems with the baby

If the baby has any abnormalities, it may lead to a miscarriage. But these are unlikely to be the cause of recurrent miscarriage.

Infection

Any infection that makes you very unwell can cause a miscarriage. Milder infections that affect the baby can also cause a miscarriage.

Shape of the uterus

It is not clear how much an abnormally shaped uterus contributes to recurrent miscarriage or late miscarriages. Minor variations do not appear to cause miscarriage, but significant fibroids and polyps are associated with recurrent miscarriage.

Long-term health conditions

Some long-term health conditions can increase the risk of having a miscarriage in the second trimester, especially if they’re not treated or well controlled. These include:

  • diabetes
  • high blood pressure
  • lupus
  • kidney disease
  • an overactive thyriod gland
  • an underactive thyriod gland.

Polycystic ovary syndrome (PCOS)

Polycystic ovary syndrome is a common condition. It is when the ovaries don’t always release an egg in the middle of the menstrual cycle (the start of a period to the start of the next one). The ovaries can become larger than normal.

Polycystic ovary syndrome is related to hormonal changes in the ovaries and it can lead to problems getting pregnant. There's also some evidence to suggest it may also be linked to an increased risk of miscarriages in fertile women.

Medications

Medicines that increase your risk include:

  • misoprostol – used for conditions such as rheumatoid arthritis
  • retinoids – used for eczema and acne
  • methotrexate – used for conditions such as rheumatoid arthritis
  • non-steroidal anti-inflammatory drugs (NSAIDs) – such as ibuprofen, which are used for pain and inflammation.

Other medicines are unsafe during pregnancy. It’s always best to ask your doctor, midwife, pharmacist or dentist about any medications that you are taking.

Find out more about drugs and medicines in pregnancy.

Food poisoning

Food poisoning is caused by eating food that contains bacteria, viruses or parasites For example, pâté may contain listeria. These are bacteria that can cause an infection called listeriosis. Listeriosis is rare, but it can cause miscarriage, harm a baby during pregnancy or cause severe illness in a newborn.

Find out more about foods to avoid in pregnancy.

Your age

Your age isn’t something you can control. But unfortunately, the risk of miscarriage does increase with age. One medical study showed that the risk of having a miscarriage is 8.9% for women aged 20-24 years and increases to 74.7% in women aged 45. 

This is because the number and quality of eggs gets lower as you get older. This is also why it can take longer to get pregnant as you get older.

The risk of miscarriage also increases with the age of the father. This is because chromosomal anomalies in sperm are found more often as men get older. Chromosomes are blocks of DNA which contain the instructions for developing every single part of a baby. Anomalies can cause genetic abnormalities in the baby, which sometimes causes miscarriage.

The risk of miscarriage is highest among couples where the woman is aged 35 years or older and the man is aged 40 years or older.

A man’s ability to father a baby also declines with age, though not as much as in women.

Myths about the causes of miscarriage

There are several myths about what can cause a miscarriage. The following things are not linked to an increased risk of miscarriage:

  • having sex (sex is safe during pregnancy unless your doctor has told you not to)
  • working
  • flying
  • eating spicy food
  • lifting or straining
  • exercising
  • your emotional state, such as being stressed or depressed
  • having a shock or a fright.

Lifestyle changes you can make

It is important to know that miscarriages very rarely happen because of something you did or didn’t do. However, there are some lifestyle choices, such as smoking, that increase your risk of having a miscarriage. There are things you can do to try and reduce this risk. These include:

It’s important to remember that there is nothing you can do that will guarantee that you won’t have a miscarriage. However, we have lots of information about what you can do to try to reduce the risk and stay as healthy as possible before you try for a baby and during pregnancy.

Was it my fault?

Unfortunately, doctors can’t always give a reason why a miscarriage happens. This can be very difficult to come to terms with. Without having a reason why, some women and couples end up unnecessarily blaming themselves for what happened. We have answered some common questions that we hope will prevent this.

Find out more about why did I miscarry and was it my fault?

Tests and treatments after miscarriage

Sometimes doctors can do tests to try to find out why you had a miscarriage and what this means for any future pregnancies.

Most women go on to have a healthy pregnancy after a miscarriage, so tests are not usually offered until a woman has recurrent miscarriages or 1 late miscarriage. This may vary in different areas, so talk to your GP or midwife to find out more.

Royal College of Obstetricians & Gynaecologists (2016) Early miscarriage https://www.rcog.org.uk/globalassets/documents/patients/patient-information-leaflets/pregnancy/pi-early-miscarriage.pdf

NHS Choices. Miscarriage https://www.nhs.uk/conditions/miscarriage/ (Page last reviewed 01/06/2018 Next review due: 01/06/2021)

NHS Choices. Foods to avoid in pregnancy https://www.nhs.uk/conditions/pregnancy-and-baby/foods-to-avoid-pregnant/ (Page last reviewed: 23/01/2017 Next review due: 23/01/2020) 

Anderson, N  et al (2000) Maternal age and fetal loss: population based register linkage study. British Medical Journal. 2000 Jun 24; 320(7251): 1708–1712.

NICE Clinical Knowledge Summaries. Miscarriage https://cks.nice.org.uk/miscarriage (Page last reviewed: May 2018 Next review due: December 2023)

Clinical Knowledge Summaries (Feb 2019) Antenatal care – uncomplicated pregnancy https://cks.nice.org.uk/antenatal-care-uncomplicated-pregnancy