A woman's fertility begins to decline in her late 20s, but her overall chances of becoming pregnant do not start to slide so soon, a new study concludes. According to a report published in the journal Human Reproduction, female fertility starts to fall off gradually around age 27 before dropping more dramatically after age 35. The findings suggest that older would-be parents may have to wait longer before becoming pregnant.

David Dunson of the National Institute of Environmental Health Sciences and his colleagues studied 782 couples who were using the rhythm method of contraception. This approach allowed the researchers to control for both the timing and frequency of intercourse. They found that women between the ages of 19 and 26 with partners of similar age had approximately a 50 percent chance of becoming pregnant during any one menstrual cycle if they had intercourse two days prior to ovulation. For women aged 27 to 34, the chance was 40 percent, and for women over the age of 35, the probability dropped to 30 percent. "Although we noted a decline in female fertility in the late 20s," Dunson notes, "what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy." He estimates that it would take women in their late 20s or early 30s a month or two longer to become pregnant than it would have required in their early 20s.

The study further found that both partners contribute to the increased time necessary to conceive because men's fertility also declines with age, though not as early as it does for women. When the team controlled for the age of women, they found that fertility was significantly reduced for men older than 35. In an encouraging find for older parents-to-be, the team reports that the length of the fertile window--the interval within the menstrual cycle during which a woman is most likely to become pregnant--did not decrease with age, holding steady at six days. Perhaps the most important thing for anxious would-be parents to remember is that fertility varies greatly from person to person. "Epidemiological studies have identified some of the factors associated with variability, including prenatal exposures, sexually transmitted disease history, smoking and occupational exposures," Dunson says, "but much of this heterogeneity remains unexplained."