Babies born by elective Caesarean section are much more likely to develop breathing problems, a Danish study examining 34,000 deliveries suggests.
Labour may help mature the lungs
Researchers found they were up to four times more likely to have respiratory problems than those born naturally, or by emergency Caesarean section.
The babies may miss out on hormonal and physiological changes during labour which help mature the lungs, they say.
The University of Aarhus study features in the British Medical Journal.
Almost a quarter of UK births are now estimated to be Caesarean sections - far above the 10% to 15% rate recommended by the World Health Organization.
More than half of these were emergency Caesareans, but despite this experts have been calling for measures to reduce numbers of elective Caesareans, warning it is a major operation.
A recent Oxford University study found that women could be four times more likely to die in childbirth if they opted for a Caesarean instead of natural birth.
The Danish team examined data on over 34,000 deliveries, adjusting to take account of factors such as the mother's age, weight, and whether she smoked or drank alcohol during pregnancy
They found that babies born by elective Caesarean section had an increased risk of general respiratory problems.
The risk was higher the earlier the Caesarean was performed.
A nearly fourfold increased risk was found at 37 weeks gestation, a threefold increase in risk at 38 weeks gestation, and a doubling of risk in infants delivered at 39 weeks gestation.
For example, at 37 weeks, 10% of babies delivered by elective Caesarean section developed respiratory problems, compared with 2.8% of infants delivered naturally or by emergency Caesarean section.
At 38 weeks, the proportion was 5.1% of elective Caesarean babies compared with 1.7% of those born naturally or by emergency Caesarean, and at 39 weeks, 2.1% compared with 1.1%.
The risks of serious respiratory problems showed the same pattern.
The researchers conclude that significantly fewer babies would develop breathing problems if elective Caesareans were put off until 39 weeks gestation.
They said: "It is plausible that hormonal and physiological changes associated with labour are necessary for lung maturation in neonates and that these changes may not occur in infants delivered by elective Caesarean sections."
Dr Maggie Blott, a consultant obstetrician at King's College Hospital, London, said obstetricians in the UK were advised not to carry out elective Caesareans before 39 weeks.
She said part of the problem might be that doctors had to switch support lines to the baby very quickly during a Caesarean, and it was possible that lung fluid is not drained away as well as it should be.
She said: "Some babies do develop transient breathing problems, they usually recover from them, but occasionally a baby can be very sick indeed.
"A lot of woman are completely unaware of the fact that a planned Caesarean section can negatively impact on their baby.
"Any research which reinforces the fact that Caesareans are not necessarily in the best interests of the baby is welcome."
Mervi Jokinen, of the Royal College of Midwives, said Caesarean section rates were too high in the UK.
She said it was a major operation, which had health implications for the mother, as well as the baby.
"The decision to opt for a Caesarean section should not be taken lightly and should be based on good medical grounds," she said.