Babies delivered by non-emergency caesarean are up to four times more likely to have breathing problems than those delivered vaginally, Danish researchers said on Wednesday.
The finding adds to evidence on the risks of elective caesareans, or C-sections, which are increasingly popular around the world.
Anne Hansen of the Aarhus University Hospital, Denmark, said the risks were greatest when a caesarean was performed early and she urged women not to choose the procedure before the 39th week of pregnancy.
"This is important information for women who choose elective sections ... we suggest you should not do elective caesareans until 39 weeks, whereas a lot of elective sections are done at 37 and 38 weeks today," she said.
Just why caesarean babies are at higher risk is unclear but Hansen said it was likely due to hormonal and physiological changes associated with labor.
The process of labor triggers the release of stress hormones in the mother, which are passed on to the fetus and are thought to be key in maturing the baby's water-filled lungs.
Cases of breathing problems associated with caesarean births are often not serious but they typically require incubator and oxygen treatment.
Hansen and colleagues based their research, which was published in the British Medical Journal, on a study of more than 34,000 births in Denmark.
They found a nearly fourfold increased risk of breathing difficulties in caesarean babies delivered at 37 weeks, a threefold increase at 38 weeks and a doubled risk at 39 weeks.
As a result, 10 percent of caesarean babies had respiratory problems when delivered at 37 weeks, compared to 2.8 percent of infants intended for vaginal delivery. By 39 weeks, the proportion was 2.1 percent compared to 1.1 per cent.
Hansen stressed that C-sections still had many benefits, particularly in the case of breech babies and where the mother or baby was in danger.
But the medical profession should do more to try and curb rising demand for non-emergency caesareans. "Most women choose it because their first delivery was a scary experience and we should really try to make sure the first delivery is a good experience," she said.