A New Bead on Birth Control

The answer to these problems would eventually come from a research project begun in 1982 by a man who came to be known as "the King of Pee."

For four years, Allen J. Wilcox, an epidemiologist at the National Institute of Environmental Health Sciences in Durham, N.C., had 221 women who hoped to become pregnant record the days of their menstrual periods and the days they had intercourse. The women also gave a daily urine sample, and Wilcox ultimately gathered more than 30,000 little jars. By testing the urine for hormones released during ovulation, Wilcox was able to determine the ovulation date for each woman each month.

Wilcox's analysis revealed the exact length of the fertile period: six days each month, including five days prior to ovulation and the day of ovulation. It turns out that sperm are hardy and can survive for up to five days in a woman's reproductive tract, but eggs are delicate and die quickly if no sperm are in the neighborhood.

And when in their cycles did the women ovulate? He found that while women with irregular cycles could be ovulating at almost any time, women with regular cycles of about 28 days followed a certain pattern: If you view Day 1 as the first day of a woman's menstrual period, the majority of women have their six-day fertile period somewhere between days 7 to 17. "Outside those days, " said Wilcox, "if you want to play the odds [and have unprotected sex] there's a low chance of getting pregnant, about 2 percent."

The risk is small, he added, "but it's not zero."

The Game of Life

All reversible methods of pregnancy prevention are about playing the odds, to a certain extent. (Among typical users of condoms, for instance, 15 percent of women became pregnant in one year, as did 8 percent of women who took an oral contraceptive, according to the forthcoming edition of Contraceptive Technology.)

So when the Georgetown institute was developing its Standard Days Method and CycleBeads, it sought to define a fertile period that would apply to most women, said Jennings.

Armed with Wilcox's findings, the Georgetown team analyzed a World Health Organization database of 7,500 menstrual cycles, including information on the date of ovulation for each cycle. They determined that about 80 percent of all women have cycles of 26 to 32 days, and 95 percent of these women will ovulate and have their six-day fertile window somewhere between days 8 and 19 of their cycles.

"The Standard Days Method isn't right for everyone," said Jennings. "It's only appropriate for women who have regular cycles of 26 to 32 days long. Women with cycles longer or shorter than that should use another method." (Indeed, if a woman using CycleBeads discovers that she has more than one cycle that is "out of bounds," the package insert instructs her to find another method of family planning.)

Among women with regular cycles of 26 to 32 days, efficacy tests published in the journal Contraception show that over the course of a year, 12 percent will become pregnant with typical use -- a rate comparable to that for diaphragms and male condoms.