By JENNY HOPE

Last updated at 08:53 12 December 2007

Babies born by planned Caesarean section are up to four times more likely to suffer breathing problems, researchers say.

They believe children delivered by Caesarean may miss out on vital hormonal and physiological changes occurring during labour and which are necessary for the lungs to develop.

The study is likely to raise concerns about the soaring number of Caesareans among mothers "too posh to push", who want the convenience of a planned operation.

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Up to one in three babies are now delivered by Caesarean section in some parts of the country - twice the 15 per cent recommended by the World Health Organisation.

Previous research has already shown that, while emergency Caesarean births can be life-saving, planned surgery is far riskier.

Mothers who elect for Caesareans are more likely to develop complications and spend twice as long in hospital as those having a natural delivery.

The study, published today online in the British Medical Journal, looked at 34,458 babies born between 37 and 41 weeks, and delivered between January 1, 1998, and December 31, 2006.

Some 2,687 of the babies were born by elective Caesarean section (7.8 per cent), 28,894 (83.9 per cent) by normal delivery, and 2,877 (8.3 per cent) by emergency Caesarean.

Factors which could affect the results, such as maternal smoking and alcohol intake during pregnancy, were also taken into account.

The study found 2.8 per cent of infants delivered normally at 37 weeks had respiratory problems compared to ten per cent of those delivered by elective Caesarean section.

At 38 weeks, the proportion was 1.7 per cent compared to 5.1 per cent. At 39 weeks, it was 1.1 per cent compared to 2.1 per cent.

Dr Hansen, of the Aarhus University Hospital in Denmark - who led

the research - said it was once thought that when a baby comes down the birth canal, the lungs are "squeezed" into reflex breathing action.

She added: "We now believe when the mother starts labour, stress hormones in the baby rise, which leads to physiological changes.

"These include the disappearance of water from the lungs and production of surfactant, a substance which allows the lungs to function properly.

"Compared with infants delivered naturally, those born by elective Caesarean section had an increased risk of respiratory problems before 40 weeks.

"A nearly four-fold increased risk was found at 37 weeks and a three-fold increase in risk at 38 weeks, whereas the risk was doubled in infants delivered at 39 weeks' gestation.

"Wherever possible, surgery should be postponed to 39 weeks in order to reduce the risk of breathing problems.

"This information should be taken into consideration by women contemplating an elective Caesarean section and by the obstetricians counselling them."

In October, an Oxford University study found women could be four times more likely to die in childbirth if they opted for a Caesarean instead of a natural birth.

The review of more than 94,000 births showed that women who had a planned Caesarean put themselves and their babies at increased risk of complications.

Known risks to the mother include bleeding and infection, while babies are more likely to need specialist care.