Study: Fertility declines in late 20s

LONDON, England -- A woman's biological clock starts ticking even earlier than was previously thought, according to new research carried out across Europe.

Doctors have found that a woman's fertility starts declining as early as her late 20s -- not, as had widely been assumed, in her mid-30s.

They also found that a man's fertility began to fade significantly in his late 30s, despite previous research that hinted male fertility starts to decline in the 40s or 50s.

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.

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Dr David Dunson, who led the Europe-wide study which involved 782 couples, said women should not worry if they have not had a child by the time they reach 30.

It simply meant they might take a month or two longer to get pregnant than if they had tried in their early 20s.

The study, reported in the journal Human Reproduction, was carried out by the National Institute of Environmental Health Sciences in North Carolina and researchers at the University of Padua, Italy.

Women taking part kept daily records of body temperature and days on which sexual intercourse and menstrual bleeding occurred.

They were divided into four age groups -- 19-26, 27-29, 30-34 and 35-39 -- and allowances were made for the differences between their ages and those of their partners.

From the data, the researchers estimated the chances of a woman becoming pregnant following intercourse on a given day.

Assuming intercourse occurred at the peak time for conception -- two days before ovulation -- and with partners the same age, women aged 19 to 26 had a 50 percent chance of becoming pregnant in any one menstrual cycle.

This fell to around 40 percent for women aged 27 to 34, less then 30 percent for women aged 35 to 39 if they and their partners were the same age. If at this stage the man was five years older, a woman's pregnancy chances fell to 20 percent.

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

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

He emphasised that the findings, only referred to the probability of women getting pregnant in any one menstrual cycle -- not the overall likelihood of pregnancy. There were a total of 433 pregnancies in the study.

Irrespective of age, there was also an enormous amount of fertility variation between different couples.

The 'super fertile' period -- the six days leading up to ovulation -- remained the same for women of all ages.

Even among the most "average" couples, the chances of a pregnancy arising from intercourse during the peak conception time ranged from 20 to 60 percent.

Although many of the factors contributing to this variability were known, the reason for it was still largely unexplained.

Dr Chris Ford, a researcher at the University of Bristol in England who studies fertility and age, told the Associated Press: "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."