Preterm Birth

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Causes and risk factors

When unplanned, it is not always clear why a baby is born preterm as most women and birthing people have no obvious risk factors. That said, there are some risk factors that make the chance of having a preterm birth more likely, including:¹

Waters breaking - called preterm prelabour rupture of membranes, or PPROM

This is when the membrane surrounding the baby that holds the amniotic fluid ruptures before 37 weeks of pregnancy. This is the most common cause of preterm birth – it happens in about 4 of every 10 preterm births.² Of those women and birthing people who experience PPROM, almost half (44%) will go on to give birth within a week.³

Having a multiple birth such as twins

If a mother or birthing person is pregnant with more than one baby, such as twins or triplets, there is a higher chance of preterm birth.⁴

Natural preterm birth happens in:⁴

58%(about 6 in every 10) of twins

~95%(19 in every 20) of triplets or higher multiples

Also, their doctor may recommend that their babies are born early for the safety of the babies. In this case, they will be offered an induced labour or a caesarean section.^5,6

Previous premature birth

If a mother or birthing person has had a preterm before, they 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.⁷ This may cause some anxiety, but your healthcare team will be aware of this risk and monitor you and your baby appropriately.

Placenta problems

Preterm birth is more likely if the placenta is low lying (less than 2 cm from the cervix) or is covering the cervix – a condition called placenta praevia.⁸
Also, if the placenta starts to come away from the wall of the womb – called placental abruption, it can cause bleeding from the vagina, tummy pains and contractions, which may lead to preterm birth.⁹

Weak cervix

In some pregnancies, the neck of the womb (the cervix) shortens and opens too soon. This is called having a weak cervix, or cervical incompetence or cervical insufficiency, and can lead to preterm birth.^10,11 The cause isn’t always clear but previously having a preterm birth or a miscarriage, or damage to your cervix, increases the risk of it happening.¹¹

Gestational diabetes

Gestational diabetes is diabetes that develops when you are pregnant and usually resolves after the birth.¹² It causes high blood sugar and symptoms of needing to pass urine more often, being very thirsty, and feeling more tired than usual. Babies who are exposed to high blood sugar levels throughout pregnancy are at a higher risk for obesity and type 2 diabetes.¹²
Having gestational diabetes makes preterm birth more likely, but most people go on to have healthy pregnancies and healthy babies. However, depending on the health of you and your baby, your healthcare team might recommend a planned preterm birth.¹³

Other factors¹³

Some demographic and lifestyle factors can also increase the chance of having a preterm birth:

Age:

women and birthing people who get pregnant in their teens/early 20s or over the age of 40.

Weight:

being underweight or overweight (for overweight women and birthing people, the risk is due to complications leading to a planned preterm birth).

Alcohol:

as well as increasing the risk of miscarriage and stillbirth, drinking alcohol during pregnancy also increases the risk of preterm birth, with higher risk the more you drink.

Smoking:

smoking is harmful to mother and baby, and it increases the risk of preterm birth. Stopping smoking is one of the best things you can do to give your baby a healthy start in life.

Ethnicity:

In England and Wales, the Black or Black-Mixed ethnic group are more likely to have a preterm birth than other ethnic groups.

Infection and inflammation:

spontaneous preterm birth is often linked with infection and inflammation. Chorioamnionitis is inflammation of the membranes around the baby in the womb, which can also happen with infection. Urinary tract infections (UTIs) can increase the risk of complications during pregnancy, including preterm birth.

Planned preterm birth

Sometimes, a health condition develops in the baby or the mother or birthing person that can lead the healthcare team to recommend a planned preterm birth, to ensure the safety of the baby and parent.¹³ These include:

Pre-eclampsia

Pre-eclampsia is a condition that affects some people during pregnancy, most often after 20 weeks or soon after the birth. Signs of pre-eclampsia* are high blood pressure (hypertension) and protein in your wee (called proteinuria). If you have pre-eclampsia, you may be advised to have your baby at about 37 weeks, but if your healthcare team are concerned about the health of you or your baby, they recommend earlier preterm birth.¹³

Foetal growth restriction (FGR)

Some babies don’t grow as much as they should during pregnancy; this is called foetal growth restriction or FGR. FGR can cause complications with your pregnancy but be assured that your healthcare team will track your baby’s growth and will advise you on the best time for your baby to be born, which may be preterm.¹³

*Defined as hypertension (high blood pressure) along with at least one of the following conditions that has arisen after 20 weeks of gestation: proteinuria (high levels of protein in the urine), dysfunction of maternal organs (which may include acute kidney injury, liver involvement, neurological issues, or haematological [blood] complications), or dysfunction of the uteroplacental unit (the uterus and placenta).¹⁴

Next: Diagnosing possible preterm birth