LONDON (Reuters) - A woman's biological clock starts ticking in her late 20s, not her mid-30s, and male fertility also begins to wane with age, doctors said Tuesday.

In what is thought to be the first study to show a drop in female fertility below the age of 30, researchers in the United States and Italy said their results do not mean older couples will not be able to conceive, it just might take them longer.

"Although we noted a decline in female fertility in the late 20s, what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy," said Dr. David Dunson of the National Institute of Environmental Health Science in North Carolina.

Actresses and celebrities such as Susan Sarandon and Madonna have had children in their 40s but doctors have long known that female fertility begins its slow decline much earlier.

Dunson's research pushes it earlier still and shows it is not just a female problem. Men also suffer from a diminishing ability to procreate.

"Fertility for men is less affected by age, but shows significant decline by the late 30s," Dunson said.

The scientists studied 782 healthy Italian couples using natural methods of contraception to determine the impact of age on conception. Their research is published in the journal Human Reproduction.

The doctors estimated the chances of conceiving during the peak time, or fertile window, in the woman's monthly cycle for couples in three age groups: 19 to 26 years, 27 to 34 years, and 35 to 39 years.

The youngest women had a 50% chance of achieving a pregnancy in any one menstrual cycle. It fell to 40% for the 27- to 34-year-olds.

For women in their late 30s, it dropped to less than 30% and if their partner was 5 years older, the chances of conceiving slumped to about 20%.

"Nearly all pregnancies fell within the fertile window and, on average, the day-specific probability declined from the late 20s onwards being around twice as high for women aged 19 to 26 as for women aged 35 to 39," Dunson said in a statement.

"When we controlled for the age of women, we found that fertility was significantly reduced for men aged over 35," he added.

Even within each group the researchers found a lot of variation in conception among the healthy couples which could not be explained by age.

Infertility affects an estimated one in six couples. Forty percent of cases are due to a male factor and an equal share is due to a female problem. In some couples the infertility is unexplained or due to a joint problem.

Reference Source 89

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