NEW YORK (Reuters Health) - The findings from a new study suggest that biological factors increase the likelihood that a woman will engage in intercourse during her most fertile days rather than at other times. Therefore, for couples not desiring pregnancy, this suggests that a single episode of unprotected sex may be more risky than chance alone would dictate.

In a variety of mammals, intercourse is coordinated with ovulation through different mechanisms, such as an increase in libido during the fertile period or by accelerating ovulation to match intercourse. In humans, however, an association between intercourse and ovulation has not been established.

As reported in the June 10th online issue of Human Reproduction, Dr. Allen J. Wilcox, from the National Institute of Environmental Health Services in Durham, North Carolina, and colleagues investigated this topic by analyzing data from 68 sexually active women.

The women, who had undergone tubal ligation or had an IUD, kept intercourse diaries for up to three menstrual cycles and provided urine samples for hormone testing, which allowed the authors to determine the day of ovulation. The 6 days ending with ovulation were defined as the fertile days. Data from a total of 171 ovulatory cycles were included in the analysis.

The authors found that intercourse frequency rose steadily during the follicular cycle and then fell off abruptly after peaking at ovulation.

The six fertile days were also the days when intercourse was most frequent, the investigators report. Compared with the other days, intercourse was 24% more frequent during the fertile days (p < 0.001).

"It is remarkable that the biological forces shaping this intimate aspect of human behavior have gone largely unrecognized, perhaps because the effect is modest in absolute terms," the researchers state. "For couples who wish to conceive, these biological mechanisms act as a silent partner that facilitates the optimum timing of intercourse."

Hum Reprod 2004;June 10th online issue:000-000.

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