Study provides most precise insight into age at which fertility starts to drop

Tuesday, April 30th 2002, 12:00 am

By: News On 6

LONDON (AP) _ New research has provided the most precise insight yet into when biological clocks start ticking loudly _ and it's sooner than once thought: age 27 for women and 35 for men.

Until now, it was thought that women's fertility starts to drop significantly in the early 30s, with a big plunge after 35. But the new study indicates that, on average, female fertility begins its meaningful slide at age 27.

And while the decline in human fertility tied to aging had traditionally been attributed to the female factor, the study, published Tuesday in the journal Human Reproduction, showed that men's fertility starts dwindling after 35.

Nevertheless, experts said the findings should not raise undue concern. The results mean it may take a month or two longer to conceive than it does for younger people, they said. The ages at which declines were seen are only averages and there is a wide range in fertility at any specific age.

``Certainly very young women in their early 20s are more fertile than women in their late 20s and early 30s. But I suspect that the fertility of those women who are around 30 is high enough that it doesn't give them a real cause for concern or worry up to the age of 35,'' said Dr. Chris Ford, a researcher at the University of Bristol in England who studies fertility and age, but was not involved with the study.

The study involved 782 healthy couples from across Europe who were using only the rhythm method of family planning. Women kept daily records of their body temperature, recorded the days they had sex and which days they had their menstrual periods.

The researchers then categorized the women into four age groups _ 19-26, 27-29, 30-34 and 35-39 _ and recorded the ages of their partners.

There were a total of 433 pregnancies.

Women in the 27-29 age group had lower pregnancy chances on average than women aged 19-26. The likelihood of pregnancy did not noticeably decline between the age groups 27-29 and 30-34, but then dropped again from age 35.

The probability of getting pregnant on any specific day in the menstrual cycle was twice as high for women under 27 as it was for women 35 and older, the study found.

Assuming that the couples had sex at the best time for conception _ two days before ovulation _ and presuming that the men were the same age as the women, women younger than 27 had a 50 percent chance of conceiving during that menstrual cycle.

This fell to about 40 percent in women aged 27-34, and after 35, it was less than 30 percent.

``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 one researcher, David Dunson, a biostatistician at the U.S. National Institute of Environmental Health Sciences.

The study also found that men's fertility dropped after 35. Previous research had hinted that male fertility starts to decline in the 40s or 50s.

The study found that while a 35-year-old woman with a partner the same age had a 29 percent chance of getting pregnant in one month, her chances dropped to 18 percent if her partner was 40.

No decline in male fertility was seen before age 35, and the man's age only seemed to matter when the woman passed 35, the study found.

At the age of 40, men were 40 percent less likely to get their partners pregnant in a month than they were at the age of 35, Dunson said.

``The observed decline is not dramatic, and you could hardly use the results to recommend males to deposit a frozen semen sample at age 35 in case they might want to be fathers at a later age,'' said Dr. Svend Juul, a professor at the Institute for Epidemiology and Social Medicine at the University of Aarhus in Denmark, who was not connected with the research.

One reassuring finding, experts said, was the super fertile period _ the six days leading up to ovulation _ remained the same for women of all ages. Some scientists have suspected that part of the reason why it is more difficult for older women to get pregnant could be that her fertile time could be shorter than that of younger women.