Women's fertility 'starts to decline in late twenties'

Women's fertility begins to decline when they reach their late twenties and not in their thirties as previously thought, according to research published today.

Women's fertility begins to decline when they reach their late twenties and not in their thirties as previously thought, according to research published today.

The study, which is the first to observe a drop in fertility before women reach the age of 30, serves as a warning against women delaying motherhood to develop their careers. Fertility declines by nearly half as women progress from their early twenties to their late thirties, according to the study, published in the Human Reproduction journal.

This means that women aged 19 to 26 are twice as likely to get pregnant during their monthly "fertile windows" as women aged between 35 and 39.

Male fertility is less affected by age. Past research has suggested that men do not have to start worrying about their biological clocks until they enter their forties, but the study shows that men's fertility can start to decline "significantly" in their late thirties and genetic defects in sperm can be seen among men in their early thirties.

Researchers from America and Italy tracked the sex lives of 782 women, aged between 18 and 40, who lived in London, Paris, Brussels, Dusseldorf, Lugano, Milan and Verona. Neither the women, nor their partners, had anyhistory of fertility problems. The women were asked to check their body temperature each day to show when ovulation occurred, record the days on which they had intercourse and when they had their periods.

At the end of the study period, there were 103 pregnancies among women aged 19 to 26, 154 among women aged 27 to 29, 140 among the 30 to 34 age group and 36 pregnancies among women aged 35 to 39.

Dr David Dunson, who led the study, said that women who delayed trying to become pregnant until their late twenties or early thirties did not have a lower overall chance of conceiving. But he said it may take a month or two longer than it would have done in their early twenties. "Although we noted a decline in female fertility in the late twenties, what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy," he said.

Dr Dunson, of the National Institute of Environmental Health Sciences in North Carolina, said most previous analyses of age-related changes in fertility could not separate the effects of reduced intercourse from the direct effects of age. But this study was "uniquely reliable" because it had in-built controls to take account of this.

The research, also carried out at the University of Padua in Italy, showed that a woman's fertile interval lasts about six days each month and ends on the day of ovulation. Encouragingly for older would-be parents, this interval did not decrease much with age, only reducing by one day when the woman was older than 35 and the man older than 40.

Age had a bigger part to play when researchers looked at the likelihood of women becoming pregnant if they had intercourse during the peak time for conception, which is two days prior to ovulation.

Women aged 19-26, who had a partner the same age, had around a 50 per cent chance of pregnancy in any one menstrual cycle. This fell to around 40 per cent for women aged 27 to 34. For women aged 35-39 it was less than 30 per cent if the woman had a partner the same age, but fell further to about 20 per cent if the man was five years older because his fertility had also started to decline.