Assisted labour during childbirth can raise the risk of autism by more than a third, research has shown.
The link is especially strong for boys and comparable with other autism risk factors such as having an older mother and early birth.
Scientists who studied data on around 625,000 births found the chances of a boy developing the behavioural disorder increased by 35 per cent if his mother underwent two forms of assisted labour.
“Inducing or augmenting labour has been previously suggested as a contributing factor to autism development. However, these studies produced conflicting results and consisted of a relatively small number of subjects,” said lead researcher Simon Gregory, from Duke University Medical Centre in the US.
“Our study is by far the largest one of its kind to look at the association between autism and induction or augmentation.”
Induced labour involves stimulating contractions before they start spontaneously Augmenting labour increases the strength, duration or frequency of contractions.
In the case of boys, both raised the risk of autism whether used together or individually.
Only augmentation alone was associated with an increased risk among girls.
But the actual likelihood of a child developing autism after an assisted birth remains very low, the researchers pointed out.
Of the 625,042 children whose births were included in the study, just 1.3 per cent of the boys and 0.4 per cent of the girls went on to be diagnosed with autism.
Two in 1,000 autism cases among boys whose mothers experienced induced or augmented labour could have been prevented if their births were unassisted, according to the researchers.
The actual likelihood of a child developing autism after an assisted birth remains very low
Further work is needed to investigate the reasons for the link, they said. But they concluded that the results, published online in the journal Jama Pediatrics, do not justify changes in clinical practice.
“The findings of this study must be balanced with the fact that there are clear benefits associated with induction and augmentation of labour,” said co-author Chad Grotegut, also from Duke University. “Labour induction, especially for women with post-date pregnancies or medical conditions such as diabetes and high blood pressure, has remarkably decreased the chance of stillbirth.”
Assisted labour has increasingly been used in recent years to prevent childbirth complications in vulnerable women.
The number of children with autistic spectrum disorders, a range of conditions that affect a person’s ability to socialise and communicate, is also said to be rising.
Around 700,000 people in the UK, or one per cent of the population, is autistic.
For the new research, the Duke University team matched information about births in North Carolina over an eight-year period with school records showing diagnoses of autism.
The researchers found that women who underwent assisted births were more likely to have autistic children than those who did not, after taking account of other risk factors such as the mother’s health and socio-economic status.
Why assisted labour increases the chances of a child developing autism is still unknown, but the scientists said that the link may not be causal. Exposure to the hormone oxytocin, which is administered to 50 per cent to 70 per cent of women undergoing induced labour, may be one explanation, they suggest.
Oxytocin signalling influences brain function and social behaviour, and has been linked to autism.
British statistician Kevin McConway, from The Open University, said: “Even if it turns out that we could save a few children from autism by inducing many fewer labours, that would come at a major cost in other complications including deaths to mothers and babies.”