Having unprotected sex once is far more likely to result in a pregnancy than was previously thought, finds research.
Better take precautions
Scientists found evidence that women are subconsciously driven to have more sex during the most fertile time of their monthly cycle.
An analysis by the US National Institute of Environmental Health Sciences found intercourse was 24% more frequent on fertile days.
The study is published in the journal Human Reproduction.
The study focused 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.
Lead research Professor Allen Wilcox 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."
It was already known that in many mammal species, intercourse is often coordinated with ovulation - but the same phenomenon had been established in humans.
Overall, the researchers found that women had sex 0.29 times a day - or twice a week.
However, during the six most fertile days of their cycle the rate of intercourse averaged out at 0.34 times a day, compared to 0.27 times a day for the rest of the month - an increase of 24%.
Professor Wilcox said there were at least three possible explanations:
- An increase in the woman's sex drive at ovulation
- An increase in the woman's sexual attractiveness
- Intercourse accelerating ovulation
He said: "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 are a caution.
"Such couples need to know that nature is subtly working against them."