In a study on women who had either been sterilised or were using an intrauterine device (IUD) 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.
The research team studied 68 sexually active women over three months (a total of 171 ovulatory cycles). The women kept diaries of days when intercourse occurred, and collected daily urine specimens. Researchers later used the samples to identify the fertile days in the cycles. Overall, intercourse was 24% more frequent during the fertile days than during the rest of the cycle.
Lead research Professor Allen Wilcox from the National Institute of Environmental Health Sciences in Durham, 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 (and we don't yet understand the biological reasons behind this) a woman who engages in a single act of unprotected intercourse is more likely to get pregnant than was previously believed."
He said: "It's not uncommon for a doctor to hear from an unhappily pregnant patient that she and her partner had taken a chance 'just this once'. It may be easy to dismiss such claims, but our data suggest these women are probably telling the truth."
Prof. Wilcox and his colleagues from the NIEHS 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 hypothesis definitively," said Prof. Wilcox.
"It's remarkable that the biological forces shaping this intimate aspect of human behaviour have gone largely unrecognized. In part, this may be because the effect on intercourse is modest. But it's also because we just haven't paid much attention."
What is the take-home message for couples? "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 are a caution. 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."
 On the frequency of intercourse around ovulation: evidence for biological influences. Human Reproduction. DOI:10.1093/humrep/deh305.
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