Nature's ambush: pregnancy more likely from single unprotected intercourse tha...( and the National Institute for Occupat...)

and the National Institute for Occupational Safety and Health in Cincinnati, Ohio, were following up on their earlier research showing six days in a menstrual cycle when a woman can become pregnant the five days up to ovulation and the day of ovulation itself. 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.

The overall frequency throughout the cycle was 0.29 per day (equivalent to twice a week). During the six fertile days it was 0.34 compared with 0.27 for the rest of the month an increase of 24%. Intercourse peaked just before and on the day of ovulation yet the women had no reason to modify the timing in relation to their fertile cycle.

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 h
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Contact: Margaret Willson
m.willson@mwcommunications.org.uk
44-0-1536-772181
European Society for Human Reproduction and Embryology
9-Jun-2004


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