Causes of premature birth

There are things that may increase the risk of premature birth, such as an infection or placental problems, but we don’t always know why it happens.

Most of the time, premature births happen spontaneously (naturally) and doctors will not know why.

Sometimes, a premature birth is planned because it's safer for mum and/or the baby. This could be because mum or baby has a health condition, such as severe pre-eclampsia, that means it is safer not to continue the pregnancy.

There are some things that increase the risk of going into labour early, so some people may be told that they have a higher risk of giving birth early.  

This page lists the things that could cause premature birth. They include your pregnancy history, the health of a current pregnancy, your medical history and who you are (eg your age, BMI and ethnicity).

Risk factors for premature birth

Previous premature birth

If you’ve given birth early before, you are more likely to give birth early again. The more premature deliveries you have had, and the earlier your babies were born, the higher this risk becomes.

“It’s important to remember that every pregnancy is different. My first baby was delivered by emergency c-section at 32 weeks. My second was a week overdue and my third arrived at 38 weeks.”

Previous late miscarriage

The causes of late miscarriage and premature birth can be the same. If you had a late miscarriage before then you have a higher chance of having a late miscarriage or a premature birth in a next pregnancy. It doesn't mean that it will happen though. The more the doctors know about the reasons for the previous experience, the more they will be able to make sure it doesn't happen again.

Low PAPP-A levels 

Pregnancy Associated Plasma Protein-A (PAPP-A) is a hormone that is made by the placenta in pregnancy. It is measured as part of the antenatal screening tests you are offered during pregnancy.

Low levels of PAPP-A may mean that the placenta (the organ that helps the baby grow and develop) may not work as well. There may be an increased chance of premature birth or pre-eclampsia

If you are diagnosed with low levels of PAPP-A you’ll likely be offered extra scans to monitor your baby’s growth. It’s also important to get to know your baby’s movements and contact your midwife or maternity unit immediately if you think your baby’s movements have slowed down, stopped or changed. Find out more about monitoring your baby’s movements.

Your midwife will also advise you to stop smoking because this can affect how your placenta works and slow down your baby’s growth. 

Placenta praevia

Sometimes the placenta attaches low down in the uterus and may cover part of or all of the cervix (the neck of the womb). In most cases, the placenta moves upwards and out of the way as pregnancy continues, but sometimes it doesn’t.

This condition is known as low-lying placenta if the placenta is less than 20 mm from the cervix or as placenta praevia if the placenta covers the cervix. If you have placenta praevia, you are at a higher risk of giving birth early.  

Find out more about a low-lying placenta

Polyhydramnios (too much amniotic fluid)

Amniotic fluid is the fluid that surrounds your baby in the womb. Polyhydramnios is where there is too much amniotic fluid around the baby during pregnancy. This slightly increases the risk of pregnancy complications, including premature birth.

We don’t always know why too much fluid builds up in pregnancy, but it can be caused by things like:

Too much amniotic fluid is normally spotted during a check-up in the later stages of pregnancy. It is not usually a sign of anything serious, but you'll probably have some extra check-ups during pregnancy. Try not to worry though as most babies whose mothers have polyhydramnios will be healthy. Speak to your doctor or midwife if you have any concerns.

Preterm premature rupture of the membranes (PPROM or waters breaking early)

If your waters broke early in a previous pregnancy then there is a small chance that this will happen again. PPROM can cause late miscarriage or premature birth.

Uterine abnormalities (unusually shaped womb)

A uterine abnormality is a womb (uterus) that formed in an unusual way before birth. Depending on the shape of the womb, there may be a higher risk of premature birth. 

Most women are unaware that they may have an abnormally shaped womb when they get pregnant. It is usually discovered during investigations for infertility or miscarriage

Find out more about uterine abnormalities

Bleeding after 24 weeks

Bleeding after the first trimester can be a sign that there is a problem with the placenta, such as a low-lying placenta or placental abruption, which can both cause premature birth.    

Any bleeding in pregnancy should be investigated, even if you are not experiencing any other symptoms. If you are more than 12 weeks pregnant and have bleeding, go to your local A&E or contact the hospital maternity unit immediately so you can be checked, just in case. 

Problem with your cervix (a weak cervix) 

Sometimes the neck of the womb (cervix) shortens during pregnancy which may indicate that you are at risk of preterm birth. Doctors do not always know why this happens, but it can happen if the cervix has been damaged. For example, from a tear in childbirth or treatment for an abnormal cervical screening, such as largo loop excision of the transformation zone (LLETZ) or a cone biopsy

When this happens, the cervix is called weak or incompetent, which means it could open or shorten early. This could lead to premature labour. If scans show that there is a problem with your cervix, you may be offered a cervical stitch (also known as a cerclage or cervical suture) or hormone treatment (such as progesterone) to stop your baby being born too soon.

Find out more about treatment for premature birth.

Multiple pregnancy (twins, triplets or more)

Twins and triplets are often born prematurely. More than 50% of twins and almost all triplets are born before 37 weeks. Even if you don’t go into premature labour, you may be advised to give birth before your due date to reduce the risk of complications. 

Some of the treatments available to delay premature birth, such as progesterone pessaries, a cervical stitch or Arabin Pessary are not routinely offered to women carrying multiple babies. Bed rest is also not routinely advised. 

You may be offered a medication called corticosteroids during your pregnancy to help your baby’s lungs develop more quickly. This can reduce the risk of complications associated with giving birth prematurely. 

Your doctor or midwife can give you more information. Find out more about multiple pregnancy.

Twin-to-twin transfusion syndrome (TTTS) 

Twins that share a placenta (monochorionic pregnancies) also share the same blood supply. In around 15 in 100 monochorionic twin pregnancies, the blood flow may be unbalanced. This means that 1 baby receives too little blood and has a low blood pressure, while the other baby receives too much blood and has a high blood pressure. This is called twin-to-twin transfusion syndrome (TTTS). 

TTTS can sometimes lead to a premature birth. If you are carrying twins, you will be monitored with frequent scans for signs of TTTS. 

The Twins Trust has more information about TTTS, including symptoms to look out for.

Type 1 and type 2 diabetes

Premature birth is one of the most common problems associated with diabetes and pregnancy. But managing your blood sugars well before and during your pregnancy, will help to reduce the risk. Find out more about type 1 and type 2 diabetes in pregnancy.

Gestational diabetes

Women with gestational diabetes are sometimes recommended early delivery by induction or caesarean section if the healthcare team are worried about the baby or mother’s health, and risk of complications.   

Some studies have also shown that some women with gestational diabetes are at higher risk of spontaneous premature birth.  

But try not to worry. Most women with gestational diabetes have healthy pregnancies and healthy babies, especially if their condition is diagnosed and managed well during pregnancy. Find out more about gestational diabetes.

Antiphospholipid syndrome (APS)

Antiphospholipid syndrome is an immune system blood disorder that can cause pregnancy complications including premature birth. If you’ve had recurrent or late miscarriages, you may be able to get referred for tests to try and find out why. This may include tests for APS. 

If you are diagnosed, treatment can improve your chances of having a successful pregnancy. You’ll also be carefully monitored carefully during your pregnancy.

If you already know you have APS, the best thing you can do is speak to your GP or specialist before you get pregnant or as soon as possible if you are already pregnant. This is because the treatment needed to give you the best chance of a successful pregnancy is most effective when it starts as soon as possible. Find out more about APS.


Pre-eclampsia is a condition that affects some pregnant women. It is a combination of raised blood pressure (hypertension) and protein in your urine (proteinuria). Pre-eclampsia usually develops after 20 weeks of pregnancy or soon after the baby is delivered. Pre-eclampsia can range from mild to severe. If the pre-eclampsia is severe, it may be recommended that the baby is delivered early. Find out more about pre-eclampsia.

Waters breaking early (PPROM)

Preterm prelabour rupture of membranes (PPROM) is when your waters break prior to labour before 37 weeks of pregnancy. 

About 50% of women with PPROM will go into labour within the first week after their waters break. The further along you are in your pregnancy the more likely you are to go into labour within 1 week of your waters breaking. PPROM is associated with 3-4 out of every 10 premature births. 

If you think your waters may have broken, you should contact your healthcare professional and go to the hospital for a check-up as soon as possible. Find out more about waters breaking early (PPROM).

Fetal growth restriction (FGR) 

Fetal growth restriction (FGR) is a condition where a baby is smaller than expected or when a baby's growth slows or stops during pregnancy. It is also called intrauterine growth restriction (IUGR). 

If your baby has FGR, there is an increased risk of complications in pregnancy. Your healthcare team will monitor the baby’s growth and wellbeing closely and you may be advised to give birth earlier than your due date. Find out more about fetal growth restriction.

Intrahepatic cholestasis of pregnancy (ICP)

Intrahepatic cholestasis of pregnancy (ICP), also known as obstetric cholestasis, is a liver disorder that can affect you during pregnancy. 

Normally, bile acids flow from your liver to your gut to help you digest food. In ICP, the bile acids build up in your body instead. ICP has no known cure except delivery of the baby.    

ICP increases the risk of premature birth. Most of these preterm deliveries are started medically (induction or caesarean), rather than happening naturally to reduce the risk of complications. Find out more about intrahepatic cholestasis of pregnancy.

Infection in the womb

Intrauterine infection is infection within the womb. Infections that could go up to the womb include E-Coli and Group B Streptococcus (GBS), bacterial vaginosis, chlamydia, trichomona, gonorrhoea, syphilis and HIV. Intrauterine infection can cause premature birth. 

Find out more about intrauterine infection.

Urinary tract infections

UTIs can cause premature birth if they are not treated. It is not an emergency if you get a UTI in pregnancy, but it should be treated as soon as possible. Find out more about symptoms and treatment for urinary tract infections

Placental abruption

Placental abruption is when the placenta starts to come away from the inside of the womb wall. It can cause stomach pain, bleeding from the vagina and contractions. This can increase the risk of premature birth.

If it is serious, or you are close to your due date, the baby may need to be born immediately (usually by caesarean section). But if you are still early on in your pregnancy and the abruption is minor, you may just be monitored closely, usually in hospital. Find out more about placental abruption.


If you become pregnant below the age of 18 or over the age of 40, there is a higher risk of premature birth. 


Black women are 3 times more likely to give prematurely than any other ethnicity. 

Obviously, there is nothing you can do about your age or race, so the best thing anyone can do if they are pregnant is to concentrate on trying to be as healthy as possible. 

It’s also important to tell your midwife or doctor if there is anything that you're worried about. Do not worry if you've talked about it before and don't be concerned about whether you're wasting anyone's time. This is your pregnancy and it's important to trust your own instincts if you feel something isn't right.

If you feel your midwife isn’t listening to you, you can ask to see another member of the team. Find out more about how to speak up in pregnancy.


Women who are not in the healthy weight range (over or under-weight) when they get pregnant are more likely to have premature birth. At your first antenatal visit, called the booking appointment, your midwife may measure your height and weight to you to work out your body mass index (BMI). Your BMI is a measure that uses your height and weight to work out if your weight is in a healthy range.

Try not to be offended if anyone involved with your health care uses the words ‘underweight’ 'overweight' or 'obese' to describe your weight. Many women don't like these terms, but nobody is judging you. The medical experts looking after you during pregnancy may need to use them so they can make sure you have the best advice and support to help you have a healthy pregnancy. 

Find out more about being underweight or overweight in pregnancy

Psychological or social stress

Some research suggests that severe stress or depression during pregnancy is linked to premature birth, although it’s not clear why. This stress may often exist alongside related issues such as lack of social support, emotional abuse or domestic violence and the things that some people do when they are severely stressed, such as eating poorly or smoking. 
It’s natural to feel a bit stressed or anxious when you’re pregnant. But if you are struggling with these feelings, you may need help.

Pregnancy can be a very emotional experience and it can sometimes be difficult to know whether your feelings are manageable or a sign of something more serious. Trust yourself. You are the best judge of whether your feelings are normal for you. 

There are no rules about how stressed you must be before talking to your midwife or GP about how you feel. You can talk to a healthcare professional at any time if you have any concerns during your pregnancy. The sooner you ask for help, the sooner you can get the right support, if you need it. 

Find out more about managing your mental health before, during and after pregnancy

Lifestyle choices that affect your risk of premature birth

There is clear evidence that your lifestyle can affect your pregnancy, so there are things you can do to try to limit the risk of premature birth.

Alcohol and premature birth

Drinking large amounts of alcohol in pregnancy increases the risk of premature birth as well as:

  • miscarriage
  • low birth weight
  • learning difficulties and behavioural problems in later life for the baby.  

When you drink, alcohol passes from your blood through the placenta and to your baby. There is no known safe level for drinking during pregnancy, so the safest approach is not to drink at all while you're pregnant. Find out more about alcohol in pregnancy and where you can access more support.

Smoking and premature birth

Smoking is the biggest cause of pregnancy problems and loss that you can change. It increases the risk of premature birth as well as:

It’s never too late to stop smoking. Every day of pregnancy that is smoke-free helps your baby’s health and development. 


Experts don't yet know enough about the safety of effects of vapes or e-cigarettes in pregnancy, so it is best to avoid using them if you can. But if using an e-cigarette helps you to stop smoking, it is much safer for you and your baby than continuing to smoke.  

But there is a better way to stop smoking than using vapes or e-cigarettes. If you need help to stop smoking in pregnancy, licensed nicotine replacement therapy products (such as patches and gum) are recommended. These products have been tested and are considered safe in pregnancy.

The best thing to do is speak to a midwife, GP, a pharmacist or an NHS stop smoking service before using any of these products. If you get support to stop smoking, your chance of quitting completely is much higher than if you try and do it alone.  

Find out how to get support to stop smoking.

Illegal or recreational drugs and premature birth

Using illegal drugs such as cannabis can increase the risk of premature birth. Find out more about illegal or recreational drugs in pregnancy.

Is there anything I can do to prevent a premature birth?

There may be some things you can do to reduce your risk of giving birth early. Find out more.

What happens if I am at risk of premature birth?

The care you’ll get if you are seen to be at extra risk of premature birth will depend on the reason why you are at risk.

For example, if you have pre-eclampsia or you are having more than 1 baby. Find out more.  

What research is being done about premature birth?

We still have a lot to learn about premature birth so it's not always possible to explain the causes and why it happens. This is partly why prevention of premature birth is still in need of extensive research – without knowing the causes, a treatment is difficult. Tommy's invests more than £400,000 annually into prematurity research.


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Review dates
Reviewed: 23 August 2021
Next review: 23 August 2024