What causes a miscarriage?

At the moment, it isn’t always possible to say what caused a miscarriage. That’s why Tommy’s opened the UK’s only miscarriage research centre dedicated to understanding miscarriage and preventing it.

Research has found some things that may cause miscarriage or make it more likely to happen. This page has information on the causes and risk factors we do know about.    

It’s very unlikely that a miscarriage was caused by something you did or didn’t do. 

If you’ve already had 1 or more miscarriages, you can use our Miscarriage Support Tool to find out more about your risk of miscarriage. The tool is based on some of the factors on this page. 

On this page:

Causes and risk factors for miscarriage

Things that do not increase your risk of miscarriage

Tests and treatments after miscarriage

Was it my fault I had a miscarriage?


Causes and risk factors for miscarriage

A cause is something that makes the miscarriage happen, for example, chromosomal abnormalities.

A risk factor is something that has been linked to miscarriage and may make it more likely to happen, for example the age of the biological parents.



A problem with the baby’s chromosomes or genes

Most early miscarriages (before 12 weeks) are caused by a problem with the baby’s chromosomes. Chromosomes are blocks of DNA with instructions for your baby’s development. They can sometimes develop abnormally. When this happens, your baby gets too many or not enough chromosomes. You baby can’t develop properly, and the pregnancy can’t continue.

Genetic factors can cause recurrent miscarriage. If one partner has an abnormality in one of their chromosomes, this may be passed on to every new pregnancy. This can cause repeated miscarriages. The partner may not be aware of this abnormality. It is rare, happening in around 2–5 in 100 pregnancies.


Problems with the placenta

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. Problems with the placenta can happen at any stage of pregnancy and can lead to miscarriage and stillbirth.


Weak cervix (cervical insufficiency)

The cervix is the entrance to the womb. To give birth, the cervix opens to allow the baby out. If the cervix is weak it opens too soon and can cause either a late miscarriage or a preterm birth.

You can read more about symptoms and treatments for a weak cervix here.


Problems with the womb lining (endometrium)

To get pregnant, an embryo needs to attach to the lining of your womb (implant). Your womb lining needs to be ready for this at the same time as the embryo.

After the embryo has attached (implanted), your womb lining needs to change again to allow the placenta to develop properly.

These changes depend on your hormones and cells in your womb lining being in the right balance at the right points in your cycle. This balance can sometimes change from cycle to cycle. 

If they are not in the right balance at the right times, you may experience fertility problems or miscarriage.


Blood clotting problems

Antiphospholipid syndrome (APS) is a syndrome that makes your blood more likely to clot. It’s not common but can cause recurrent miscarriage and late miscarriage. Find out more about tests and treatments for antiphospholipid syndrome.

Thrombophilia is an inherited condition that means that your blood may be more likely to clot. This may cause late miscarriages. Treatment for inherited thrombophilia has not been found to improve chances of a healthy pregnancy.


Tommy's research into the causes of miscarriage

Risk factors

Risk factors are not the same as causes. Some people who have all the risk factors will not have a miscarriage and some people who have none of them will still have one.  

Risk factors are things that have been linked to miscarriage, but we don’t know if or how they cause one to happen.


Shape of the womb (uterus)

Some people (around 5-6 in 100) have wombs that are unusually shaped. 

In women and birthing people who have recurrent miscarriage, it is more common to have a uterus that is an unusual shape (around 13 in 100 women).

Small differences in shape do not appear to cause miscarriage. But others are more likely to be a risk factor, such as a septate uterus and a bicornuate uterus. You can read more about unusually shaped wombs here.


Long-term health conditions

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

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

Our Planning for Pregnancy tool can give you tailored advice if you are planning a pregnancy and have a long-term health condition, including a mental health condition


Polycystic ovary syndrome (PCOS)

Polycystic ovary syndrome is a common hormone condition. It affects around 1 in 10 women and birthing people. It can slightly increase your risk of miscarriage.



There are some medicines that increase your risk of miscarriage. These include:

  • misoprostol – used for conditions such as rheumatoid arthritis and to terminate pregnancies
  • 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.

There are other medicines that are unsafe during pregnancy. It’s always best to ask your doctor, midwife, pharmacist or dentist about any medications that you are taking before trying for a baby.  

Find out more about drugs and medicines in pregnancy.

It’s important to balance the risks for your baby of taking a type of medication while pregnant with the risks for you and your baby of not taking the medication. There isn’t always a right or easy answer.

Do not stop taking prescribed medication without talking to your doctor, even if you are already pregnant. This includes medicine you might be taking for mental health conditions.  

BUMPS (Best Use of Medicines in Pregnancy) has lots more information about different types of medication and the research that has been done into the risks and benefits.


Food poisoning

Food poisoning is caused by eating food that contains bacteria, viruses or parasites. This can increase the risk of miscarriage.  

For example, pâté may contain listeria. These are bacteria that can cause an infection called listeriosis. Listeriosis can harm a baby during pregnancy or cause severe illness in a newborn baby.

Not all foods to avoid in pregnancy will automatically give you food poisoning. It is recommended to avoid them because they are foods that are more likely to contain harmful bacteria, viruses or parasites.

It’s important to remember that even if you have been ill, this does not necessarily mean that your illness caused you to miscarry – it is likely to be a coincidence.  

Find out more about foods to avoid in pregnancy.


Your age

Unfortunately, the risk of miscarriage increases as the biological parents get older.

This is because the number and quality of eggs gets lower as you get older. The risk of miscarriage also increases with the age of the biological father. This is because problems with the chromosomes in the sperm are found more often as men get older.

Risk of miscarriage by age of the woman or birthing person:  

  • under 20 years old: 16 in 100 will miscarry
  • 20 to 30 years: 12 in 100 will miscarry
  • 30 to 34 years:  14 in 100 will miscarry
  • 35 to 39 years: 18 in 100 will miscarry
  • 40 to 44 years: 37 in 100 will miscarry
  • over 45 years: 65 in 100 will miscarry

Find out more about miscarriage statistics


Your ethnicity

Most Black women have successful pregnancies and health babies. But for complex reasons, Black women and birthing people have an increased risk of miscarriage compared to White women.

More research is needed to help us understand why. We are trying to change this by making sure that Black women and birthing people are represented in our research into the causes of miscarriage.  

You can read more about ethnicity and miscarriage risk here.

If you Black or Black Mixed-Heritage, and are planning a pregnancy, already pregnant or have had a loss, we have a 1:1 support line for you to talk about anything that is worrying you.


Air pollution

There is a growing amount of research showing that air pollution can have an impact on the health of your pregnancy. Some types of air pollution (carbon monoxide and particulate matter) may increase the risk of miscarriage.

Most people cannot change where they live but you might be able to limit your exposure to air pollution in other ways.


Previous miscarriages  

If you’ve had 1 or more miscarriages already, your risk of having another increases slightly with each one. If you have had a live birth in between 1 or more miscarriages this decreases your risk of another miscarriage.

You can find out your risk of another miscarriage based on previous miscarriages and 4 other factors using our Miscarriage Support Tool. It uses an algorithm devised by the Tommy’s National Centre for Miscarriage Research to accurately calculate risk.


Being over or underweight

People with a BMI of under 19 or over 25 have an increased risk of miscarriage. The risk is higher if your BMI is over 30.

Read more about managing your weight while planning for pregnancy and during pregnancy.


Drinking, smoking, too much caffeine and taking illegal drugs

Research has shown us that miscarriages are more likely to happen to women or birthing people who:

It's worth remembering that, even if you did any of these things, your miscarriage could have been caused by something completely different. But you can help reduce your risk of miscarriage in the future by stopping or reducing the amount you do these things.

If you drank small amounts of alcohol before you realised you were pregnant, the risk of harm to the baby is low. Many women drink alcohol early in their pregnancy because they didn’t realise they were pregnant and go on to have healthy babies. It is highly unlikely that your miscarriage was caused by a couple of drinks.  

Drinking heavily during pregnancy can increase your risk of miscarriage. The more you drink, the greater the risk of harm to your baby.


Very high levels of stress

It’s possible that very high levels of stress may be linked to miscarriage. Our page on stress and miscarriage has more detail. 


Things that do not increase your risk of miscarriage

The following things are NOT linked to an increased risk of miscarriage.

Having sex  

Sex during pregnancy is safe unless your doctor or midwife has told you not to. If your pregnancy is normal with no complications, having sex and orgasms won’t cause a miscarriage.

You will be advised to avoid sex if:

  • your waters have broken
  • there are any problems with the entrance to your womb (cervix)
  • you’re having more than 1 baby or have had early labours before - and are in the later stages of pregnancy.
  • your placenta is covering the entrance to your womb (a low-lying placenta).

If you or your partner are having sex with other people during your pregnancy, it’s important to use a barrier form of contraception, such as a condom. Sexually transmitted infections such as chlamydia and gonorrhoea may cause miscarriage.

Find out more about sex and pregnancy.



Flying during pregnancy is safe up to a certain point if you are having an uncomplicated pregnancy. There is no evidence that flying can cause miscarriage (or early labour or your waters to break).

We don't advise flying later in pregnancy because, if you do go into labour in the air, you would not have the right medical support.

Find out more about flying in pregnancy.


Vaccines – flu, whooping cough, Covid-19

There is no evidence that recommended vaccinations can cause miscarriage. Getting the flu in pregnancy can cause complications such as bronchitis, pneumonia, premature birth and even stillbirth. Public health authorities in the UK recommend that every pregnant woman has the flu jab, whooping cough and Covid-19 vaccinations. 

Find out more about vaccines in pregnancy.



There is no evidence to suggest that occasionally using saunas, jacuzzies, hot tubs and steam rooms during pregnancy causes miscarriage.  

However, when you use a sauna, jacuzzi, hot tub or steam room, your body is unable to lose heat effectively by sweating. This means your body's core temperature rises and it's possible that a significant rise in your core temperature could be harmful in pregnancy, particularly in the first 12 weeks. Some research has shown that a rise in your body’s core temperature (hyperthermia) increases the risks of neural tube defects, such as spina bifida.

The NHS recommends avoiding them because of the risks of overheating, becoming dehydrated and fainting. 


A previous abortion

Having an abortion does not increase your risk of having a miscarriage, ectopic pregnancy or low-lying placenta. However, you may have a slightly higher risk of premature birth.

Find out more about abortions and future pregnancies.



There is no evidence that exercise causes miscarriage.

It’s understandable to worry that your baby is shaken around as you exercise, but this is not the case. Babies are secure inside the womb. Staying active in pregnancy is important for your health and the health of your pregnancy and baby.  

Unless your doctor or midwife has recommended otherwise, start or continue to keep active in pregnancy.

Find out more about exercise and pregnancy.


Normal levels of stress

Some stress is a normal part of life and does not seem to be linked to miscarriage. Throughout history, humans have had babies successfully in all kinds of very stressful situations.

Our page on stress and miscarriage has more detail.


Tests and treatments after miscarriage

Tests are not usually offered until a woman or birthing person has recurrent miscarriages (3 miscarriages) or 1 second trimester loss (late miscarriage).  This is because most women and birthing people go on to have a healthy pregnancy after a miscarriage.

Find out more about tests and treatments after miscarriage here.


Was it my fault I had a miscarriage?

Not knowing why you have had a miscarriage can be hard. Without having a reason, some people end up blaming themselves for what happened.  

In almost all cases, it will have nothing to do with anything you did.

If you have already had 1 or more miscarriages, we have a Miscarriage Support Tool. This calculates your risk of another based on some of the factors on this page.

If you are having symptoms of a miscarriage (bleeding or stomach pain), see our information on what to do if you think you are having a miscarriage right now.



NHS (2022) Miscarriage. Available at https://www.nhs.uk/conditions/miscarriage/ (Accessed 23 January 2024) (Page last reviewed: 9 March 2022 Next review due: 9 March 2025)

Royal College of Obstetricians & Gynaecologists (2016) Early miscarriage. Available at https://www.rcog.org.uk/globalassets/documents/patients/patient-information-leaflets/pregnancy/pi-early-miscarriage.pdf (Accessed 23 January 2024) (Page last reviewed 09/2016)

Royal College of Obstetricians & Gynaecologists (2023) Recurrent miscarriage. Available at https://www.rcog.org.uk/for-the-public/browse-our-patient-information/recurrent-miscarriage/ (Accessed 23 January 2024) (Page last reviewed 06/2023)  

Siobhan Quenby et al for the ALIFE2 Block Writing Committee and ALIFE2 Investigators, Heparin for women with recurrent miscarriage and inherited thrombophilia (ALIFE2): an international open-label, randomised controlled trial, The Lancet, (June 2023) DOI: https://doi.org/10.1016/S0140-6736(23)00693-1

 Muter Joanne, Kong Chow-Seng, Brosens Jan J.The Role of Decidual Subpopulations in Implantation, Menstruation and Miscarriage. Frontiers in Reproductive Health 3 2021  https://www.frontiersin.org/articles/10.3389/frph.2021.804921   DOI=10.3389/frph.2021.804921    ISSN=2673-3153

Jan Joris Brosens et al (2017) Clearance of senescent decidual cells by uterine natural killer cells in cycling human endometrium eLife 6:e31274

Lucas, E.S., Vrljicak, P., Muter, J. et al. Recurrent pregnancy loss is associated with a pro-senescent decidual response during the peri-implantation window. Commun Biol 3, 37 (2020). https://doi.org/10.1038/s42003-020-0763-1 

 Polycystic ovary syndrome - NHS (www.nhs.uk)

 Recurrent Miscarriage (Green-top Guideline No. 17) | RCOG (accessed January 2024) (last reviewed June 2023)

Li D, Liu L, Odouli R. Exposure to non-steroidal anti-inflammatory drugs during pregnancy and risk of miscarriage: population based cohort study BMJ 2003; 327 :368 doi:10.1136/bmj.327.7411.368

Nybo Andersen, A et al (2000) Maternal age and fetal loss: population based register linkage study. BMJ. 2000 Jun 24; 320(7251): 1708–1712

Siobhan Quenby, Arri Coomarasamy et al. Miscarriage matters: the epidemiological, physical, psychological, and economic costs of early pregnancy loss

H. Lashen, K. Fear, D.W. Sturdee, Obesity is associated with increased risk of first trimester and recurrent miscarriage: matched case–control study, Human Reproduction, Volume 19, Issue 7, 1 July 2004, Pages 1644–1646, https://doi.org/10.1093/humrep/deh277

Nielsen A, Hannibal CG, Lindekilde BE, Tolstrup J, Frederiksen K, Munk C, et al. Maternal smoking predicts the risk of spontaneous abortion. Acta Obstet Gynecol Scand. 2006; 85(9): 1057–65.

Clinical Knowledge Summaries (Feb 2019) Antenatal care – uncomplicated pregnancy 

NHS Highland (nd) Alcohol and pregnancy. Available at https://www.nhshighland.scot.nhs.uk/your-services/all-services-a-z/maternity-and-neonatal/enhanced-midwifery-support/alcohol-and-pregnancy (Accessed 23 January 2024)

Sundermann AC, et al. Alcohol Use in Pregnancy and Miscarriage: A Systematic Review and Meta-Analysis. Alcohol Clin Exp Res. 2019 Aug;43(8):1606-1616. doi: 10.1111/acer.14124. Epub 2019 Jul 3. PMID: 31194258; PMCID: PMC6677630.

Schliep KC, Hinkle SN, Kim K, Sjaarda LA, Silver RM, Stanford JB, Purdue-Smithe A, Plowden TC, Schisterman EF, Mumford SL. Prospectively assessed perceived stress associated with early pregnancy losses among women with history of pregnancy loss. Hum Reprod. 2022 Sep 30;37(10):2264-2274. doi: 10.1093/humrep/deac172. PMID: 35972454; PMCID: PMC9802052.

NHS (2021) Sex in pregnancy. Available at https://www.nhs.uk/pregnancy/keeping-well/sex/ (Accessed 23 January 2024) (Page last reviewed: 16 March 2021 Next review due: 16 March 2024)

Gupta NK, Bowman CA. Managing Sexually Transmitted Infections in Pregnant Women. Women’s Health. 2012;8(3):313-321. doi:10.2217/WHE.12.16

The Royal College of obstetricians and gynaecologists (2018) Air Travel and pregnancy https://www.rcog.org.uk/globalassets/documents/patients/patient-information-leaflets/pregnancy/air-travel-pregnancy.pdf

Moretti ME et al. (2005) Maternal hyperthermia and the risk for neural tube defects in offspring: systematic review and meta-analysis. Epidemiology. 2005 Mar;16(2):216-9.

 Moretti ME et al. (2005) Maternal hyperthermia and the risk for neural tube defects in offspring: systematic review and meta-analysis. Epidemiology. 2005 Mar;16(2):216-9.

 NHS Choices. Is it safe to use a sauna or jacuzzi if I’m pregnant? https://www.nhs.uk/common-health-questions/pregnancy/is-it-safe-to-use-a-sauna-or-jacuzzi-if-i-am-pregnant/  Moretti ME et al. (2005) Maternal hyperthermia and the risk for neural tube defects in offspring: systematic review and meta-analysis. Epidemiology. 2005 Mar;16(2):216-9.

 NHS Choices. Is it safe to use a sauna or jacuzzi if I’m pregnant? https://www.nhs.uk/common-health-questions/pregnancy/is-it-safe-to-use-a-sauna-or-jacuzzi-if-i-am-pregnant/  

Review dates
Reviewed: 09 February 2024
Next review: 09 February 2027