Women are more likely to get pregnant through one-off unprotected sex than previously thought, because of their subconscious urge to have intercourse on their most fertile days, research has found.

The scientists who carried out the study, published in the journal Human Reproduction, also said that a woman's sexual attractiveness might increase during ovulation through subtle behavioural clues.

They warned that while many couples might believe that a one-off session without contraception is unlikely to result in a pregnancy, the study has shown they should be much more cautious.

The researchers from the National Institute of Environmental Health Sciences in Durham, North Carolina, studied 68 women who had either been sterilised or were using an intrauterine device (IUD) to stop them getting pregnant.

The women kept diaries of when they had sex over three months and the researchers collected urine samples to identify their fertile days.

They discovered that intercourse was 24% more frequent during their six most fertile days than during the rest of their cycle. Sexual activity peaked on the day of ovulation, despite the fact none of the women wanted to conceive.

Researcher Allen Wilcox said there were biological factors promoting intercourse during a woman's most fertile days, whether they wanted a baby or not. "Sex apparently does not happen randomly. It's more likely to occur on fertile days, even though the average woman won't know when these days are."

If biological mechanisms affect the timing of intercourse, it was likely that sex would be more frequent on the most fertile days. Prof Wilcox said for those who wanted a baby, these were a "silent partner" helping them pick the best time to have sex.