How Many Days Are You Fertile?

I’m not sure where this article originally appeared, otherwise I would surely cite it. The print-out was given to me years ago by some clinician and I still refer to it as one of my most concise explanations of fertility timing. It’s a brief summary of an excellent study done by the New England Journal of Medicine: Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby

 & Health

How many days are you fertile?
Until now the majority of studies to determine when a woman is most fertile have suggested that this period lasts from three to four days before ovulation until two to three days after, although some studies show a longer “window” of ten days per cycle. A new government study, however, establishes a shorter and different fertility window—crucial information regardless of whether you’re trying to get pregnant or avoid pregnancy. All women who conceived during the study did so within a six-day period in their cycles, ranging from five days before ovulation through the day of ovulation. No pregnancies occurred when intercourse followed ovulation (see chart). The study isn’t definitive, but the tests used were more accurate at confirming timing of ovulation and conception than were those previously available.

The researcher, from the National Institute of Environmental Health Sciences, recruited 221 women to provide daily urine specimens and keep records of each day’s sexual activity. The researchers established the dates of ovulation in the 625 menstrual cycles by analyzing 27,000 urine specimins. One issue they addressed was how frequently couples should have sex when they’re trying to conceive: The study showed that intercourse every other day during the fertile period was nearly as successful in achieving pregnancy as daily sex was.

The study also has implications for women using home-testing kits to determine when ovulation is about to take place. If a woman waits until she’s about to ovulate to have sex, she has missed all but the end of her fertile period. In light of this study, kit manufacturers may need to revise their enclosed directions, which suggest that women have sex on the days following ovulation to up their chances for pregnancy. Women who want to avoid pregnancy should know that using any method to pinpoint ovulation—including monitoring basal body temperature or cervical mucus—is imprecise. “Even with the new findings, these women should allow some margin of error.” says the study’s author, Allen J. Wilcox, M.D., Ph.D., “and avoid unprotected sex for at least three days after they think they’ve ovulated.”

The chances of conception
6 days prior
5 days prior
4 days prior
3 days prior
2 days prior
1 day prior
Day of ovulation
1 day after
Adapted from data in The New England Journal of Medicine (December 7, 1995)

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