The experts say this doesn't mean older couples can't conceive, reports CBS News Correspondent Sam Litzinger, but it may take them longer, perhaps an extra month or two.
It's believed to be the first study to show a drop in female fertility below the age of 30. The study is the work of researchers in the United States and Italy and published Tuesday in the British journal Human Reproduction.
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.
"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.
And while the decline in human fertility tied to aging had traditionally been attributed to the female factor, the study showed that men's fertility starts dwindling after 35.
"Fertility for men is less affected by age, but shows significant decline by the late 30s," Dunson said.
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.
The scientists studied 782 healthy Italian couples using natural methods of contraception — that is, only the rhythm method — to determine the impact of age on conception.
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.
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 youngest women had a 50 percent chance of achieving a pregnancy in any one menstrual cycle. It fell to 40 percent for the 27- to 34-year-olds.
For women in their late 30s it dropped to less than 30 percent and if their partner was five years older, the chances of conceiving slumped to about 20 percent.
"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.