Pregnancy risk from one-off sex 'greater than thought'

The risk of women becoming pregnant is more likely occur following a single act of unprotected sex than previously believed, according to research published today.

The findings published today in the journal Human Reproduction, indicate that women subconsciously had sex more often during the six most fertile days of the menstrual cycle – and sexual activity peaked at ovulation.

The researchers studied the sexual habits of 68 women who had either been sterilised or were using an intrauterine device (IUD) – that is women who were not seeking to conceive. They found that the women were pre-disposed to have more sex during the crucial period of increased fertility.

The research team studied the women over three months (a total of 171 ovulatory cycles), and 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 researcher 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."

Prof Wilcox said there were at least three possible explanations: an increase in the woman's libido at ovulation; 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; and that Intercourse may accelerate ovulation.

For couples who want a baby these biological mechanisms are a silent partner, helping to optimise the timing of intercourse, Prof Wilcox said.

He added: "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."

(gmcg)