What causes a 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.

 

Medications

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:

• smoke
• drink alcohol or use illegal drugs, such as cocaine or cannabis
• have more than the recommended amount (200mg) of caffeine.

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

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.

 

Saunas  

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.

 

Exercising  

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.