Overdue babies are more likely to be stillborn, a major scientific review of birth records has found.
Queen Mary University of London researchers analysed 15million pregnancies, including some in the UK and US.
They found any pregnancy that extends beyond 37 weeks carries a higher risk of the baby being stillborn.
And the risk gets greater for every week that passes. However, the academics behind the study said the risk was low in real terms.
For instance, the risk for women who are 41 weeks pregnant is the equivalent of one additional stillbirth for every 1,449 pregnancies.
Experts believe the risk is higher later in pregnancy because the placenta is older and ‘less able to deliver the oxygen and nutrients needed to sustain a baby’.
Researchers found the risk of stillbirth increased in the latest stages of pregnancy, from 0.11 stillbirths per 1,000 pregnancies at 37 weeks to 3.18 at 42 weeks
Figures from the Office for National Statistics show there were around 679,000 live births across England and Wales in 2017.
Around one in five labours are induced in the UK. Most are induced if the baby is overdue or there is a risk to either the mother or baby’s health.
In the UK, induction is offered to all women who don’t go into labour naturally by 42 weeks. The risk of stillbirth is higher if gestation exceeds this.
The researchers, who were led by Professor Shakila Thangaratinam, analysed data from 13 separate studies in the UK, US, Denmark and Norway.
The statistics they analysed included 15,124,027 pregnancies, 17,830 stillbirths and 2,348 newborn deaths between 1990 and 2017.
Writing about their results, the researchers said mothers-to-be should be told of the ‘small but significantly increased’ risk.
They added: ‘There is a need to assess the acceptability of early delivery at term to parents and healthcare providers to avoid the small risk of stillbirth.’
Professor Thangaratinam added: ‘While there is an additional risk of stillbirth at 41 weeks, compared to 40 weeks, it is small.
‘Women who prefer not to have medical interventions such as induction of labour may therefore acknowledge this small additional risk.’
He said they may, therefore, ‘choose to wait until 41 weeks so that they have more time to go into labour naturally’.
‘Meanwhile, other women may prefer to have discussions with their healthcare providers on induction after 40 weeks.’
Professor Thangaratinam added: ‘So this is all about helping women make informed decisions on timing of delivery.’
Of the 3,000 babies stillborn every year in the UK, a third appeared healthy at 37 weeks, the results showed.
The risk of stillbirth increased with gestational age, from 0.11 stillbirths per 1,000 pregnancies at 37 weeks to 3.18 at 42 weeks.
The risk was 0.16 at 38 weeks, 0.42 at 39 weeks, 0.69 at 40 weeks and 1.66 at 41 weeks.
Between week 40 and 41, there was a 64 per cent increase, according to the results published in the journal PLOS Medicine.
The researchers said impaired placental function in babies carried for longer periods was likely to blame for most cases.
Experts welcomed the findings of the study, and agreed the aging of the placenta was the most likely explanation.
Dr Alexander Heazell, senior clinical lecturer in obstetrics, University of Manchester, said: ‘It is an important piece of work.
‘The placenta ages as pregnancy continues. So by the end of pregnancy it is less able to deliver the oxygen and nutrients needed to sustain a baby.’
Professor Christoph Lees, a trustee of the International Society of Ultrasound in Obstetrics and Gynecology, said the risk ‘remains very small’.
He added: ‘Hence waiting until 41 weeks before offering induction of labour may not offer complete reassurance.
Dr James Doidge, of the Great Ormond Street Institute of Child Health, said more research is needed into the benefits of inducing labour earlier.
However, he added: ‘But [the study] is insufficient grounds for changing practice or decisions about inducing labour at this stage.’
Jane Brewin, chief executive of Tommy’s, said: ‘Stillbirth rates in the UK are still unacceptably high and many countries in Europe have lower rates than the UK.
‘We welcome this study which will help inform NHS care and importantly women should be made aware of the increased risk so they can promptly report any concerns.
‘However, stillbirth is a complex issue and gestational age risks must be combined with other maternal risk factors and explained to all women.’
Dr Joy Leahy, statistical ambassador, Royal Statistical Society, said: ‘This study demonstrates that for babies who are carried to term, the likelihood of being stillborn increases the longer the baby remains in the womb.
‘However, it does not distinguish between babies who were induced and those who weren’t, which makes it difficult to use this study for decision making.
‘As a statistician, who also happens to be in my third trimester of pregnancy, it still doesn’t answer the question of whether I should proactively seek to be induced when I reach 37 weeks, or whether I should simply be hoping that my baby comes naturally before 42 weeks.’
Despite finding an increased risk of stillbirth, they found no such heightened danger of the infant dying in their first month of life.
They found babies born after 42 weeks gestation were 87 per cent more likely to die in the first month of life than those born after 41 weeks.
However, the risk of a baby dying in its first few weeks outside the womb remained the same between 38 and 41 weeks of gestation.
Stillbirths happen after 24 completed weeks of pregnancy. They occur in around one in every 200 pregnancies, the NHS states.
The cause of most stillbirths is a mystery but some can be caused by bleeding, pre-eclampsia, problems with the umbilical cord or an infection.