Women Want Sex Most On Fertile Days

14 June 2004
Women Want Sex Most On Fertile Days
by George Atkinson

Research published in the journal Human Reproduction suggests that a single act of unprotected intercourse is more likely to lead to an unwanted pregnancy than was previously believed.

The study looked at women who had either been sterilised or were using an intrauterine device (IUD) to avoid pregnancy. The frequency of intercourse increased during the six most fertile days of the menstrual cycle and peaked at ovulation - despite the fact that these women clearly did not want a baby.

Lead researcher Professor Allen Wilcox from the National Institute of Environmental Health Sciences in North Carolina said: "There apparently are biological factors promoting intercourse during a woman's six fertile days, whether she wants a baby or not. It suggests that couples who 'take a chance' with unprotected intercourse have the deck stacked against them. Intercourse apparently does not happen randomly. It's more likely to occur on the fertile days, even though the average woman won't know when these days are. For whatever reasons, a woman who engages in a single act of unprotected intercourse is more likely to get pregnant than was previously believed."

It's already known that, in mammals, intercourse is often coordinated with ovulation. This can be caused, for example, by fluctuations in libido or by ovulation being accelerated by intercourse. But none of these mechanisms has been established in humans.

The researchers hypothesised that if biological mechanisms affect the timing of intercourse, it was likely that intercourse would be more frequent during the fertile days. Their analysis of the women's data showed that, indeed, the six consecutive days with the highest frequency of intercourse corresponded exactly with the fertile days.

Prof. Wilcox said there were at least three possible explanations:

• An increase in the woman's libido at ovulation - a previous study has suggested that women have heightened interest in sex at this time
• An increase in the woman's sexual attractiveness, via subtle behavioural cues from the woman or possibly due to the production of pheromones at this time - both factors have been suggested in previous studies
• Intercourse accelerating ovulation - experiments on rodents have suggested this, and a sub-analysis of the new study supports this possibility. However, a clinical trial would be necessary to establish such a mechanism in humans. "A simple randomized trial of volunteer couples using non hormonal birth control could test this last hypothesis definitively," said Prof. Wilcox.

"It's remarkable that the biological forces shaping this intimate aspect of human behaviour have gone largely unrecognized. For couples who want a baby these biological mechanisms are a silent partner, helping to optimize the timing of intercourse. For couples who do not want a baby, however, these data sound a cautionary note. Such couples need to know that nature is subtly working against them," said Prof. Wilcox. He advises women who do not wish to become pregnant to be aware that occasional unprotected intercourse may be more risky than chance alone would predict. "There is no substitute for reliable birth control."

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