Finding means chance of pregnancy more likely than thought

Women who find themselves "in the mood" usually are at their most fertile, according to research that found that women are more sexually active during ovulation than at other times of the month.

The researchers worked with women who had been sterilized or who used an intrauterine device to prevent pregnancy, finding that the women had sex more often during the six most fertile days of the menstrual cycle, peaking at ovulation.

"Among women who were using effective birth control and not paying attention to the timing of intercourse, they were much more likely to have intercourse during the six days in their cycle when they were fertile," lead researcher Dr. Allen Wilcox, a senior investigator with the National Institute of Environmental Health Sciences, told HealthDay .

"Our finding suggests that women who are trying to get pregnant are having a little help from nature that they probably aren't aware of," Wilcox said. "So that's a good thing for couples who want to get pregnant."

But for couples who don't want to get pregnant, the finding means that women who have sex once in a while without birth control are taking a bigger chance than they may realize, Wilcox said.

"It's just at those times when they are most motivated that they are most likely to conceive," he said. "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."

Wilcox's team studied 68 sexually active women over three months -- a total of 171 ovulatory cycles. Women kept a diary of when intercourse occurred and collected daily urine samples. Using the urine samples to identify the fertile days, the researchers found intercourse increased 24 percent during ovulation, compared with the other days of the monthly cycle.

Wilcox speculated that the increase in the frequency of intercourse in humans is because of hormonal changes that affect a woman's sex drive -- the same as in other mammals.

"There are also hormonal shifts that affect the female's production of pheromones that affect the libido of males," he said. "Human biology is constructed to make couples more interested in intercourse during the very time they are likely to conceive."

Dr. Harvey J. Kliman, a research scientist in obstetrics and gynecology at Yale University School of Medicine, said he's sure the study's findings are valid.

"Having three children and a wife and watching her for 28 years, I know she is more interested at different times -- there's no question about it," he told.

Kliman noted that animals function the same way.

"We are more sophisticated than animals, but we have many overlaps," he said. "Our systems work to optimize reproduction."

But, as Wilcox said, "The moral of the story is, if you don't want to conceive, you really should use good birth control."