This could be because hormonal and physiological changes in pregnancy are vital for proper maturation of infant lungs.
For the study, outcome data on 34,000 live singleton births at 37 to 41 weeks were analysed.
In 2,687 cases, the babies were delivered by elective caesarean. The other infants were intended for vaginal delivery and born that way, or by emergency caesarean.
Problems such as respiratory distress were most likely in children born by elective caesarean.
In those born at 37 weeks, the risk of respiratory problems was 3.9 times higher in those in the caesarean group than those in the vaginal delivery group. Adjustment for birth complications had little effect on the association.