A large-scale study involving thousands of woman found that it was more likely that conceiving could take more than a year if the woman had the operation.

Record numbers of UK women are opting to have babies by caesarean section rather than naturally, and emergency caesareans are also more common.

There is no evidence that this increase has made the process of giving birth any safer for either mothers or their babies.

Longer wait

The Bristol University researchers found that 7% of women who had not had a previous caesarean found it took more than a year to get pregnant again.

Neither the medical profession nor women themselves realise the extent of the long-term problems caesarean sections can cause

Professor James Walker, RCOG

However, 12% of women with previous caesareans found it took this long to conceive.

There was no indication that caesarean patients were more likely to fail to conceive altogether.

However, the time it takes to conceive is a sensitive measure of any subtle effect on fertility.

Research needed

It is not known how the operation might be having this effect - but the research team suggested it might even be underestimating the effect.

Dr Deirdre Murphy, who led the project, said: "We looked at the delayed ability to conceive of women who did eventually go on to get pregnant.

"This may be underestimating the true magnitude of the association.

"It is possible that some women will choose not to have a further pregnancy because of the trauma of the section or will fail to achieve any further pregnancy."

She called for further research into the issue - saying that women had the right to know the full consequences of the operation before opting to undergo it.

Professor James Walker of the Royal College of Obstetricians and Gynaecologists (RCOG) said: "This is not the first study to indicate that caesarean section may affect a woman's subsequent ability to conceive.

"Neither the medical profession nor women themselves realise the extent of the long-term problems caesarean sections can cause.

"When doctors and mothers assess the risks of caesareans they generally only think about what the risks are at the time and ignore the impact they might have five years down the line."

The study was part of the "Children of the 90s" project, which looks at the development of 14,000 families since the early 1990s.