A preterm birth, one that happens before 37 completed weeks of pregnancy, is the number one cause of newborn deaths and the second leading cause of deaths in children under five.

Premature birth is a delivery which occurs before 37 completed weeks of pregnancy. The World Health Organisation gives the following definitions for the different stages of preterm birth:

  • extremely preterm: before 28 weeks
  • very preterm: from 28 to 32 weeks
  • moderate to late preterm: from 32 to 37 weeks

Globally, more than 1 in 10 pregnancies will end in preterm birth (and this number is rising). In the UK, around 80,000 babies are born each year needing specialist hospital care this is 1 in 9 babies.

In babies born preterm, the chance of survival at less than 23 weeks is close to zero, while at 23 weeks it is 15%, at 24 weeks 55% and at 25 weeks about 80%.

The length of gestation typically reduces with each additional baby. On average, most singleton pregnancies last 39 weeks, twin pregnancies 36 weeks, triplets 32 weeks, quadruplets 30 weeks and quintuplets 29 weeks. Almost 60% of twins are born preterm, while 90% of triplets are preterm.

Causes of Preterm Birth

  • 25% of preterm births are planned because the mother and/or baby are suffering life-threatening complications such as pre-eclampsia, kidney disease or growth restriction
  • 20% are due to waters breaking early (premature rupture of the membranes)
  • 25% are due to an emergency event, for example, placental abruption (when the placenta detaches itself from the uterus), infection, eclampsia or prolapsed cord (when the umbilical cord exits the body before the baby)
  • In 40% of cases the cause is not known

Survival Rates and Long-Term Outcomes of Preterm Birth

In England, survival rates of very premature babies increased from 53% in 2006 to 80% in 2011.

Survival increases by 9.5% for each week the baby stays in the womb if the baby is born at around 23 weeks, and 16% per week if the baby is born at around 25 weeks.