In a striking new study of hundreds of women trying to become pregnant, only those who had sex during a six-day interval in their monthly cycle succeeded, with the odds of conception peaking on the day of ovulation and dropping sharply after that.
This new estimate cuts in half the generally accepted two-week window of maximum female fertility. And it nearly doubles the number of days per month when it is highly unlikely for a woman to conceive.
Applying unparalleled scientific precision to an age-old question, the findings may help many couples time sexual intercourse, whether they are hoping to achieve or avoid pregnancy. The shorter, more clearly defined fertility window offers strong support for so-called natural family planning, known as the "rhythm method," the only birth control sanctioned by the Catholic Church.
The findings, the researchers said, will have immediate impact on healthy couples who have nonetheless failed to conceive after months of trying. Previously, such couples have been counseled to have sex over the several days spanning ovulation. But now it's clear that much of that effort will be wasted, the authors said, adding that couples should focus on the six days up to and including ovulation.
Moreover, the researchers found that the timing of intercourse had no bearing on the baby's gender, powerfully contradicting the popular notion that sex on the day of ovulation raises the likelihood of having a boy.
"A beautiful piece of work," said James Trussell, director of the Office of Population Research at Princeton University. Among others things, he said, "it's the first study to find that the probability of conception on the day after ovulation is zero."
"This is a real advance," said Dr. Daniel Mishell Jr., chairman of obstetrics and gynecology at USC's School of Medicine. "It confirms when people who are infertile have the highest likelihood of becoming pregnant" and also "enhances natural family planning methods."
In the study, which appears today in the New England Journal of Medicine, researchers at the National Institute of Environmental Health Sciences closely followed 221 healthy women trying to become pregnant. The women kept a daily diary of sexual activity, and daily urine samples were tested for hormones signifying ovulation and pregnancy.
Analyzing the timing of the 192 pregnancies, the researchers found that a woman had a 1-in-10 chance of conceiving if she had sex five days before ovulation; the odds rose steadily to 1 in 3 on the day of ovulation.
That none of the women conceived by having sex the day after ovulation doesn't mean it's impossible to become pregnant then, said Dr. Allen J. Wilcox, the lead author of the study. But he said that this was the strongest evidence yet for the phenomenon. He speculated that the egg is not viable for more than half a day, or that the female reproductive system abruptly becomes less hospitable to sperm.
Earlier studies failed to observe the fertility drop-off, he said, because they did not use such accurate methods for pinpointing ovulation and correlating it with sexual activity.
He downplayed the study's impact on natural family planning, primarily because there's no reliable test outside a lab to know when ovulation will begin. Currently, women using the method abstain around the onset of ovulation. They estimate this by home urine tests or measurements of body temperature, which begins to rise a day and a half before ovulation, peaking when the ovary releases the egg.
But, as the study shows, women are fertile five days before ovulation begins, Wilcox said. "What researchers need to provide is an easy way of predicting ovulation a week ahead of time," he said.
Still, others say the new research appears to bolster such abstinence-based methods, which are 80% to 97% effective in preventing pregnancy. "We welcome anything that helps us work within the design that God gave us," said Teresa Notare, a family planning specialist with the National Conference of Catholic Bishops.
Mishell said that a couple relying on the woman's body temperature to signal when to abstain will benefit from knowing that fertility drops right after ovulation. "Once her temperature stays up for a couple days, she should be able to have unprotected sex without worrying about becoming pregnant," he said.
In effect, the study nearly doubles the number of days when a woman is at minimal risk of pregnancy, from about 11 to 19, not counting the menstrual cycle.
Use of the so-called morning-after pill also may be affected by the new research, said Dr. Karen Sue Himebaugh, medical director of Planned Parenthood Los Angeles. The drug is taken the day after unprotected sex to prevent implantation of a fertilized egg. Himebaugh said many women consider using the drug only right before ovulation, whereas the new research indicates it may be warranted a few days sooner.
As for the ability of parents to select the gender of their child, the researchers found no significant association between the timing of intercourse leading to conception and the likelihood of having a boy. "There seems to be no practical value in trying to time intercourse" to select a male or female, Wilcox said.