London - Babies delivered by Caesarean section are a third more likely to develop autism later in life, researchers have claimed.
The controversial conclusion comes from data of about 20 million births worldwide examined by scientists in Sweden.
They suggest that children born via C-section have a 33% increased chance of autism as well as a 17% higher risk of attention-deficit hyperactivity disorder (ADHD).
But their research was criticised by experts who said the data was skewed. They insisted the risk can be explained by the underlying problems causing women to have surgical deliveries rather than the procedure itself.
Although the operation is often needed to avoid complications, a growing body of evidence suggests it may affect the child later in life. The Swedish academics said babies born via C-section are not exposed to bacteria in the birth canal. This could stop them developing a robust immune system and push up the risk of neuro-developmental disorders.
Another explanation could be that babies in a surgical birth do not experience the vital "stress response" involved in a natural delivery, added the study.
The academics at the Karolinska Institute in Stockholm said: "Both elective and emergency Caesarean deliveries were associated with increased odds of ADHD and autistic spectrum disorder."
Although they speculated about the biological reasons for the higher risk, the scientists admitted these "remain unknown and require empirical investigation".
British scientists said the more likely explanation is that many C-sections are for babies born prematurely, an established risk for developmental issues. Problems with the placenta, which often trigger a surgical birth, could also be to blame.
Experts advised mothers-to-be not to be concerned by the study in the JAMA Network Open medical journal.
Consultant obstetrician Dr Pat O’Brien, for the Royal College of Obstetricians and Gynaecologists, said: "Women who have a Caesarean birth should be reassured that it is a safe procedure.
"A Caesarean birth can be a life-saving intervention as well as the right choice for mother and baby."
Kevin McConway, emeritus professor of applied statistics at the Open University, said: "There are very likely to be other differences between Caesarean and vaginal births to do with the mother, the child or something else."
He added that the absolute increase in risk is tiny, up from seven babies in 1 000 with autism for natural births to about 10 per 1 000 for Caesarean deliveries.