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Sex more likely when women are most fertile
Updated: 2004-06-10 07:53

Women are likely to have more sex on the days when they are most fertile, even if they are not trying to get pregnant, scientists said on Thursday.

That means that taking a chance and having sex without using contraception could be more likely to result in unwanted pregnancies than previously thought.

"There apparently are biological factors promoting intercourse during a woman's six fertile days, whether she wants a baby or not," said Professor Allen Wilcox, of the National Institute of Environmental Health Sciences in Durham, North Carolina.

But he and his colleagues do not understand the reasons and doubt that women are even aware of them.

In a study of 68 women who recorded when they had sex over a three-month period, Wilcox and his colleagues discovered that during ovulation, when a woman is most likely to become pregnant, the overall frequency of sex increased by 24 percent.

"There are some biological mechanisms at work here that curiously we know very little about," he said in an interview.

"It suggests that couples who 'take a chance' with unprotected intercourse have the deck stacked against them. Intercourse does not happen randomly," Wilcox, who reported the findings in the journal Human Reproduction, added.

He and his colleagues are not sure why sex is more frequent during a woman's most fertile time of the month, but they suspect there are a few factors at work.

Most women have about six fertile days a month -- the five days preceding ovulation and the day of ovulation.

A woman's libido may be heightened when she is ovulating so she has an increased interest in sex, or she may produce more pheromones, chemicals that send messages to other individuals, that increase her sexual attractiveness.

Animal studies have also shown that having sex accelerates ovulation.

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

But he said women who do not want to conceive should be aware that unprotected sex may be riskier than the odds suggest.

"For couples who don't want to conceive the message would be that when you take a chance that chance is even bigger than you might think."

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