Couples trying to conceive a baby have only six days a month of reliable fertility, a narrower window of opportunity than previously thought, a new study shows.
And contrary to popular belief, the timing of sexual intercourse has no reliable effect on the sex of the baby, the researchers found.
The 221 women in the study became pregnant only during the six days up to and including the day of ovulation, when the egg is released from the ovary.
While the findings could have implications for family planning if confirmed by further research, medical specialists said they would not change any current recommendations for how to conceive or avoid pregnancy.
“If you’re talking about natural family planning methods, this is more or less what we’ve practiced all along,” said Dr. Curtis Cetrulo, director of obstetrics at St. Elizabeth’s Hospital in Boston. “This study found a narrower window of fertility than previous studies, but we wouldn’t change any of our recommendations based on one study.”
“It’s just one study,” agreed Dr. Henry Klapholz, vice-chair of obstetrics and gynecology at Beth Israel Hospital in Boston. “It doesn’t settle the debate.”
Researchers from the National Institute of Environmental Health Sciences in North Carolina studied 221 healthy women between 1982 and 1985 who planned to become pregnant. They monitored the day of ovulation by measuring estrogen and progesterone hormones in urine specimens collected by these women and followed the 192 women who eventually became pregnant.
The records showed that the probability of conception was highest, at 33 percent, on the day of ovulation itself, and sank to 10 percent with intercourse five days before ovulation.
Thus, even perfect timing won’t ensure pregnancy, according to the researchers.
In a departure from previous findings, none of the women in this study became pregnant more than five days before or a day or two after ovulation. A multicenter study by the World Health Organization found 10 fertile days per cycle, and another recent study by researchers at Baylor College of Medicine found that a few pregnancies occurred when intercourse was more than six days before ovulation and a few when intercourse followed ovulation.