FRIDAY, June 11, 2004 (HealthDayNews) -- Women are more likely to get pregnant from just one act of unprotected intercourse than previously thought.
The reason: They're more sexually active during ovulation than other times of the month, a new study finds.
The researchers worked with women who had either been sterilized or who used an intrauterine device to prevent pregnancy. They found the women had more frequent intercourse 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," said lead researcher Dr. Allen Wilcox, a senior investigator with the National Institute of Environmental Health Sciences.
"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. So that's a good thing for couples who want to get pregnant," he said.
For couples who don't want to get pregnant, the news is that women who occasionally have sex without birth control are probably taking a bigger chance than they realize, Wilcox said.
"It's just at those times when they are most motivated that they are most likely to conceive. 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," he added.
Wilcox's team studied 68 sexually active women over three months -- a total of 171 ovulatory cycles. Each woman kept a diary of when intercourse occurred and collected daily urine samples, according to the study that appears in the June 10 issue of Human Reproduction.
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 the same as in other mammals and is due to hormonal changes that affect a woman's libido. "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," Wilcox added.
Dr. Harvey J. Kliman is a research scientist in obstetrics and gynecology at Yale University School of Medicine. Commenting on the new research, he said, "From a personal level, I am sure it's true."
"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 said.
Kliman noted that animals function much the same way. "We are more sophisticated than animals, but we have many overlaps," he said. "Our systems work to optimize reproduction."
Dr. Rogerio A. Lobo, director of Reproductive Endocrinology/Infertility at Columbia-Presbyterian Medical Center in New York City, added, "I think it is an expected finding. In fact, I thought this had been done already."
"In general, mood and libido are improved during an estrogen-positive phase of the cycle (before ovulation) and is dampened thereafter -- possibly an effect of progesterone," Lobo said. "This is nature's way of facilitating reproduction."
Added Wilcox: "The moral of the story is if you don't want to conceive you really should use good birth control."
Planned Parenthood can tell you more about birth control.