Babies born through a planned caesarean section are up to four times more likely to suffer from breathing problems in the first days of life, according to a study today.
The research, published online by the British Medical Journal, shows that the earlier the caesarean is carried out, the higher the risk. Many caesareans are timetabled to take place after 37 or 38 weeks of pregnancy so that the woman does not go into spontaneous labour at 39 or 40 weeks.
"More and more women request a caesarean without any obvious reason," said lead author Dr Anne Hansen, from the perinatal epidemiology research unit at Aarhus University hospital in Denmark, where the research was carried out.
"This study tells them that there is a risk. You should know that there is a risk that your child will be in an incubator for a few days and you should put your foot down and say I don't want this at 37 weeks - I want to wait until 39 weeks," she said.
Doctors believe that surges of hormones as labour begins may kickstart the baby's lungs. Studies have shown that when labour is not initiated, the foetus does not experience an increase in catecholamine levels, a group of hormones such as adrenaline which are released by the adrenal glands at times of stress.
The babies at highest risk of breathing problems were those born by caesarean at 37 weeks, when their lungs would also not be as well developed as they would be if left to term (40 weeks). At 37 weeks, 10% of the babies born by planned caesarean had breathing difficulties, compared with 2.8% of those born vaginally. At 39 weeks, the proportions were 2.1% and 1.1%.
Hansen said there was no good reason why elective caesareans were carried out at 37 weeks.
"It is just tradition," she said. "In our study we had 190 electives done at 37 weeks. Only eight were done because the foetus was threatened. The others could have waited."
Sometimes an early operation might be carried out to suit the couple, "for instance if the husband was going away on business", but often it would be to do with the hospital and consultants' schedules. "The reasons are many, but we should not do them," Hansen said.
While some elective caesareans are carried out because of the wishes of the mother, others are done for medical reasons, which include the size of the baby and the danger that a scar from a previous caesarean section will re-open under the strain of labour. In cases where labour is potentially hazardous, a caesarean will be advised and planned by doctors. Even so, said Hansen, it may be best not to do it before 39 weeks.
"A lot of obstetricians would say we can't wait because the woman will go into labour and then we will have to do the caesarean as an emergency at night, which is higher risk for the woman," she said.
She would like to see a proper randomised trial carried out to establish what the risks and benefits are for the baby and the mother of waiting until 39 weeks in those circumstances.
For the current study, the researchers analysed the outcomes of more than 34,000 singleton babies born without birth defects in Aarhus University hospital between January 1998 and December 2006. Of those, 2,687 were born by elective (planned) caesarean section. Those born through emergency caesarean, after labour had begun, were grouped with the vaginal deliveries for the purpose of the study.
In the analysis, the study took account of the babies who were being delivered by caesarean because of a risk to their health.
The researchers found that there was a higher risk not only of general breathing difficulties in babies delivered by caesarean section, but also of serious breathing problems. Those with general respiratory difficulties would spend between one and four days in an incubator with extra oxygen in the neonatal ward. The more serious cases would be on a ventilator for some time.
Hansen said they had not investigated whether there were any long-term effects on the children.