June 9, 2004 -- Your parents or sex education teacher may have told you that it only takes a single act of unprotected sex to make a baby, but new research suggests it may be much more likely than they thought.
A new study shows that sexual activity tends to peak during a woman's most fertile time, which means the chances of becoming pregnant from a single unprotected sex act are higher.
In the study, researchers examined patterns of sexual activity in relationship to ovulation. They found that sex was 24% more frequent during the most fertile days of the women's monthly cycle.
"There apparently are biological factors promoting intercourse during a woman's six fertile days, whether she wants a baby or not," says researcher Allen Wilcox of the National Institute of Environmental Health Sciences in Durham, N.C., in a news release.
"It's not uncommon for a doctor to hear from an unhappily pregnant patient that she and her partner had taken a chance 'just this once,'" says Wilcox. "It may be easy to dismiss such claims, but our data suggest these women are probably telling the truth."
The results appear in the current issue of Human Reproduction.
Sex Peaks During Women's Fertile Days
The study involved 68 sexually active women who kept a diary of sexual intercourse and provided daily urine samples for three months. All of the women were using an intrauterine device or had had a tubal ligation to protect against pregnancy.
Previous research has shown that the five days leading up to ovulation and the day of ovulation are the days in a woman's menstrual cycle when she is most fertile. Increasing the frequency of sexual intercourse during these days increases the chance of pregnancy.
Researchers say that in mammals, this fertile period is often coordinated with intercourse through fluctuations in libido, and in some animals, ovulation is happens as a result of intercourse. But neither of these biological mechanisms has been established in humans.
However, this study suggests that some of those same forces of nature may also affect the sexual behavior of men and women.
Researchers found sexual intercourse peaked just before and on the day of ovulation among this group of women, even though they did not wish to become pregnant. The overall frequency of sex was 24% higher on the women's six most fertile days of the month compared with the rest of the days of the cycle.
The overall frequency of sex throughout the cycle was about twice a week. But during the six fertile days it was about two and a half times per week compared with slightly less than twice a week for the rest of the month.
"It suggests that couples who 'take a chance' with unprotected intercourse have the deck stacked against them," says Wilcox. "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."
Wilcox offers three possible explanations for the findings based on previous studies:
- An increase in the woman's libido at ovulation
- An increase in the woman's sexual attractiveness due to subtle behavioral cues from the woman or possibly due to the production of pheromones during ovulation
- Intercourse accelerating ovulation
"It's remarkable that the biological forces shaping this intimate aspect of human behavior have gone largely unrecognized," says Wilcox. "In part, this may be because the effect on intercourse is modest. But it's also because we just haven't paid much attention."