Giving birth by caesarean section could make it harder for women to get pregnant again, British researchers warn today.

The findings will stoke the debate over the sharp rise in caesarean births in Britain in recent decades. They have trebled in 25 years - every fifth birth in England and Wales is now a caesarean.

The study suggests women who have caesarean births are almost twice as likely to have to wait for more than a year before they can get pregnant again. Trying to conceive for more than a year is a standard measure of whether a couple have fertility problems.

Previous studies of whether caesarean births make women less fertile have failed to factor in the post-caesarean women who choose not to try to have more children.

The research, drawing on the monitoring of the health and development of more than 14,000 families in Avon since the early 1990s, allows for this and other factors which could distort the conclusions.

The study, published in the journal Human Reproduction, will be discussed at today's European Society of Human Reproduction and Embryology conference in Vienna.

Researchers from Bristol University found that women who tried to conceive again after a caesarean had a 12% risk of tak ing more than a year to get pregnant. Women who had given birth naturally had a 7% risk.

The rise in caesarean births in Britain does not appear to have benefited mother or baby. It seems to be partly motivated by more cautious doctors, fearful of lawsuits and partly by mothers, or doctors, drawn to the apparent speed and convenience of a caesarean compared to normal birth.

Deirdre Murphy, the obstetrician who led the Bristol study, said more research was needed so that mothers tempted to choose a caesarean for non-medical reasons knew what they were getting into.

"I think it's important that women are aware of the risk, particularly if they're making the choice for themselves without medical indications.

"If there is a medical indication, you have to accept the risks. If it is for their own personal preference, they need to be aware that if they're planning to have two children in quick succession there may be a delay of one or three years."

More than 60% of caesarean sections in England and Wales are classed as emergency procedures.

The researchers looked at women who eventually became pregnant after caesareans. Dr Murphy said a broader study, taking in women who didn't, might show caesarean-related fertility problems to be more severe still.