What are the causes of premature (preterm) birth?

Risk factors for premature birth

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

What happens if I am at risk of premature birth?

Premature birth, also called preterm birth, is when a baby is born before 37 weeks of pregnancy.

Most of the time, premature births happen spontaneously (naturally) and often doctors will not know why.  Researchers at Tommy’s National Centre for Preterm Birth Research are working to find out more about the causes of premature birth and how to prevent it.  

Sometimes a premature birth may need to be planned because it’s safer for the baby to be born early. This could be because of a health condition you have (such as pre-eclampsia), or that your baby has (such as fetal growth restriction). ​   

Some things are known to increase the risk of preterm birth, so you may be told that you have a higher risk of giving birth prematurely. They include your past pregnancy history, your medical history and you and your baby’s health in this pregnancy. There are also some things that you may be able to do something about, to reduce your risk. 

Risk factors for premature birth

Previous premature birth

If you’ve given birth preterm 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 the risk.

“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.”
Paula

Placenta praevia

Sometimes the placenta attaches low down in the womb and may cover part or all of the cervix (the neck of the womb). Usually it moves out of the way later in pregnancy. But sometimes it does not. It is known as low-lying placenta if the placenta is less than 20 mm from the cervix or placenta praevia if the placenta covers the cervix.  

This puts you at a higher risk of giving birth preterm, especially if you have placenta praevia.

Placental abruption

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

If you are near your due date, your baby will need to be born straight away. If you are much earlier in your pregnancy and the abruption is minor, you may kept in hospital for close observation.

Read more about placental abruption.

Polyhydramnios (too much amniotic fluid)

Amniotic fluid surrounds your baby in the womb. Polyhydramnios is when there is too much of it. Complications are rare but can include premature birth. 

We don’t always know why too much fluid builds up in pregnancy, but possible causes include: 

• being pregnant with more than 1 baby
• gestational diabetes  
• infection
• your baby has a swallowing problem
• your baby has a genetic condition.

Most people do not know they have it until it’s found at a routine antenatal appointment. You’re unlikely to need treatment for it but may have extra appointments for the rest of your pregnancy.

Uterine abnormalities (differently shaped womb)

Some people have a womb (uterus) that is different in shape or size from what is typical. This is called a congenital uterine abnormality. Depending on the type and severity, it can increase the risk of premature birth

Read more about uterine abnormalities. 

Vaginal bleeding

Call your maternity unit or GP straight away if you have any bleeding from your vagina.

Bleeding in the first trimester of pregnancy increases the risk of preterm birth compared to people who don’t have bleeding. It’s important that your midwife or doctor knows you have had bleeding so they can give you any care you need.  

Bleeding after 24 weeks of pregnancy is linked to up to 1 in 5 preterm births. It can be a sign that there is a problem with the placenta, such as a low-lying placenta, placenta praevia or ​placental abruption. 

Weak cervix (cervical incompetence or cervical insufficiency) 

Sometimes the neck of the womb (the cervix) shortens and opens too soon in pregnancy. This is called having a weak cervix, or cervical incompetence or cervical insufficiency. It can lead to your baby being born prematurely.  

Doctors don’t always know why this happens, but things that increase the risk include previous preterm birth or miscarriage. You may also be more at risk of preterm birth  if your cervix has been injured. For example, from treatment for abnormal cells found on cervical screening, such as LLETZ (largo loop excision of the transformation zone) or a cone biopsy.

In the UK, LLETZ is the most common way to remove abnormal cervical cells. It offers a balance between the risk of having abnormal cervical cells again and of premature birth in future pregnancies. But the right treatment for you will depend on your own situation and preferences and the doctor should discuss these with you. Read more about making decisions.

If you are at risk of giving birth prematurely because of a weak cervix, you may be offered treatment with a cervical stitch (also known as cervical cerclage) or the hormone progesterone to help prevent this.​ ​A cervical stitch is an operation where a stitch is put round your cervix to try to keep it closed.  

If you’re pregnant and have had treatment for abnormal cervical cells in the past, let your midwife or doctor know at your booking appointment. They will talk to you about whether you need any extra care during your pregnancy.   

Being pregnant with more than 1 baby

If you are pregnant with more than 1 baby they are more likely to be born prematurely. This can either be because you naturally go into labour prematurely or because your healthcare team recommends that your babies are born early.

Some treatments to delay​ premature birth, such as a cervical stitch or Arabin Pessary, are not routinely offered to people who are pregnant with more than 1 baby​​.  

You may be offered a medication called corticosteroids (steroids) to help your baby’s lungs develop more quickly.

Your doctor or midwife can give you more information. Read more about being pregnant with more than 1 baby.

Twin-to-twin transfusion syndrome (TTTS) 

Babies who share a placenta also share the same blood supply. Sometimes 1 baby gets too little blood and the other gets too much. This is called twin-to-twin transfusion ​​syndrome (TTTS). It increases the risk of preterm birth.

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.

Gestational diabetes

Gestational diabetes is diabetes that develops when you are pregnant and usually goes away after your baby is born.

Most people with gestational diabetes have healthy pregnancies and healthy babies. But they are also more likely to give birth prematurely. You might be advised to have a planned preterm birth, depending on you and your baby’s health.

Read more about gestational diabetes.

Antiphospholipid syndrome (APS)

Antiphospholipid syndrome or APS, sometimes called Hughes syndrome, is an immune system ​disorder that increases your risk of blood clots. It can cause pregnancy complications including premature birth due to problems with the placenta linked to blood flow or eclampsia/pre-eclampsia.

You may be tested for APS if you’ve had recurrent early miscarriages, an unexplained late miscarriage or one or more premature births. Treatment with medication to reduce the chance of blood clots forming can improve your chances of having a successful 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. It’s also important that you are only given treatment that is safe for your baby.

Read more about APS.

Pre-eclampsia

Pre-eclampsia is a condition that affects some people in pregnancy (usually after 20 weeks) or soon after their baby is delivered. Signs of it are high blood pressure (hypertension) and protein in your wee (proteinuria).

If you have pre-eclampsia, you’ll probably be advised to have your baby at about 37 weeks but it could be earlier if your healthcare team is concerned about you or your baby

Read more about pre-eclampsia.

Waters breaking early (PPROM)

Your baby is surrounded by amniotic fluid or ‘waters’ in a membrane bag (amniotic sac) in your womb. Preterm prelabour rupture of membranes (PPROM) is when your waters break​ ​before 37 weeks of pregnancy but you haven’t yet gone into labour. If this happens you have an increased risk of giving birth prematurely.

If you think your waters may have broken, contact your maternity team straight away.  

Read more about waters breaking early (PPROM).

Fetal growth restriction (FGR) 

Some babies who aren’t growing well have what’s known as fetal growth restriction (FGR).

If your baby has FGR, you are more likely to have pregnancy complications. Your healthcare team will monitor the baby’s growth and talk to you about when it would be best for your baby to be born. You may ​be advised​ to give birth prematurely.

Intrahepatic cholestasis of pregnancy (ICP)

Intrahepatic cholestasis of pregnancy (ICP), sometimes called obstetric cholestasis, is a condition affecting your liver in pregnancy. Bile acids made in the liver build up in your body instead of going to your gut to help you digest food.

If you have ICP your baby is more likely to be born prematurely. This could happen naturally or you may be offered a planned birth by induction of labour or c-section. This can happen from 35 weeks, depending on the level of bile acids in your blood and any other problems, such as pre-eclampsia.

Read more about intrahepatic cholestasis of pregnancy.

Chorioamnionitis

Chorioamnionitis is inflammation of the membranes around the baby in the womb, sometimes with infection too. Spontaneous preterm birth is often linked with infection and inflammation.

Read more about chorioamnionitis.

Urinary tract infections (UTIs)

Urinary tract infections (UTIs) can increase the risk of pregnancy complications including premature birth. 

If you get a UTI in pregnancy​ ​it should be treated as soon as possible. Limited evidence suggests that antibiotic treatment for UTIs may reduce the risk of preterm birth and low birth weight.

Read more about symptoms and treatment for urinary tract infections. 

Your age

If you become pregnant in your teens or early 20s or over the age of 40, there is a higher risk of premature birth than for people between those ages. This is sometimes due to a higher risk of pregnancy complications, such as chorioamnionitis in younger people and pre-eclampsia in older people. There is also a higher risk of spontaneous preterm birth at younger and older ages.

Ethnicity

In England and Wales, babies in the Black ethnic group are the most likely to be born prematurely.

If you are Black or Black-Mixed heritage, it is even more important to tell your midwife or doctor if there is anything that you're worried about. We run a Black and Black Mixed-Heritage helpline. If you have any concerns book a call and we will support you.  

Do not feel you're wasting anyone's time. This is your pregnancy and it's important to trust your instincts if you feel something isn't right.

FiveXMore is a Black maternal health organisation. Read their 6 steps to help you speak up for yourself in pregnancy. 

Being overweight or underweight

Being a healthy weight before getting pregnant can help reduce your risk of complications. Being underweight in pregnancy makes it more likely that you will have a spontaneous preterm birth. Being overweight makes it more likely that you will give birth prematurely, usually because of pregnancy complications leading to a planned preterm 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). BMI is a measure that uses your height and weight to work out if your weight is in a healthy range. It’s not a perfect way to measure weight, as it measures weight not fat. Someone may be muscular and have a high BMI but not much fat. But it is still useful for most people.

We know words like underweight, overweight and obese can be hard to hear. But healthcare professionals are not judging you. They may need to use them so they can make sure you have the best advice and support to help you have a healthy pregnancy. Do tell your midwife if being weighed and talking about your weight is difficult for you so they can support you. 

Read more about being underweight or overweight in pregnancy. 

Stress

Some research suggests that stress during pregnancy may increase the risk of premature birth. This stress may often exist alongside other things that increase the risk, such as eating poorly, smoking, using recreational drugs and alcohol, and domestic violence.

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 support.

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.  

Read more about managing your mental health before, during and after pregnancy. 

Alcohol

Drinking alcohol in pregnancy increases the risk of miscarriage, stillbirth and premature birth. It can also affect your baby’s growth and development and their health at birth and later on. The risks increase the more you drink.

When you drink alcohol, some passes through the placenta to your baby. There is no known safe level for drinking during pregnancy, so not drinking alcohol is the safest approach. 

Find out more about alcohol in pregnancy and where you can access more support and our tips for an alcohol-free pregnancy.

Smoking

Smoking is harmful to you and your baby. It increases the risk of problems including preterm birth. Protecting your baby from tobacco smoke is one of the best things you can do to give your child a healthy start in life and stopping smoking will help your baby straight away.

We don’t yet know the long-term effects of vaping. There has not been much research into the effects of vaping in pregnancy and it‘s not known whether the vapour is harmful to babies in the womb. Vapes are not recommended for non-smokers, but they are less harmful than cigarettes and can help you stop smoking.

Nicotine replacement products like patches and gum are licenced to use in pregnancy. 

Read more about vaping during pregnancy, smoking and pregnancy and help to stop smoking.

Recreational drugs

Using recreational, street or illegal drugs can increase the risk of premature birth.

 Find out more about illegal or recreational drugs in pregnancy and how to get help.

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. These include making healthy choices where you can about things like exercise and eating, not smoking or drinking alcohol, and going to your antenatal appointments so you get the care and support you need. Find out more.

What happens if I am at risk of premature birth?

The care you’ll get to reduce your risk of giving birth early will depend on why you are at risk. For example, if you have ​pre-eclampsia or ​you are having more than 1 baby.  You may need extra monitoring, treatment to help prevent preterm labour and support to make healthy changes, such as stopping smoking.​

Find out more.