WOMAN'S biological clock starts ticking in her late 20s, not her mid 30s, and male fertility also begins to wane with age, doctors reported.

In what is thought to be 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 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 old, 27 to 34 years old and 35 to 39 years olds. The youngest women had a 50 percent chance of achieving a pregnancy in any one menstrual cycle. It fell to 40 per cent for the 27 to 34 year olds.

For women in their late 30s it dropped to less than 30 per cent and if their partner was five years older, the chances of conceiving slumped to about 20 per cent, "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 19-26 as for women aged 35-39", Dunson said in a statement. -Reuters.