Guidelines for getting pregnant or avoiding pregnancy
usually assume an average woman is fertile between days 10 and 17 of
her menstrual cycle. But researchers at the National Institute of
Environmental Health Sciences have demonstrated what some accidentally
pregnant women may have long suspected:
Only about 30 percent of women actually have their fertile
window entirely within that timespan.
In fact, the researchers found, there is hardly a day in the
menstrual cycle during which some women are not potentially fertile.
Women in this study were of prime reproductive age (most between 25
and 35) when the menstrual cycles are most regular. The window of
fertility would be even more unpredictable for teenagers or for women
approaching menopause, the NIEHS researchers said.
Describing data on tests during nearly 700 menstrual cycles of
213 healthy North Carolina women, NIEHS' Allen J. Wilcox, M.D., Ph.D.,
statistician David Dunson, Ph.D., and epidemiologist Donna Day Baird,
Ph.D., reported in the British Medical Journal (Vol. 321, pp
1259-1262) that "women should be advised that the timing of their
fertile window can be highly unpredictable, even if their cycles are
Dr. Wilcox, a physician and chief of epidemiology at NIEHS,
said, "If the average healthy couple wants to get pregnant, they are
just as well off to forget 'fertile windows' and simply engage in
unprotected intercourse two or three times a week."
As for those seeking to use the cycle to avoid pregnancy, the
odds are not good. "Our data suggest there are few days of the
menstrual cycle during which some women are not potentially capable of
becoming pregnant including even the day on which they may expect
their next menses to begin," according to the new scientific report.
With cycle day one being the day of the actual onset of
menstrual bleeding, the researchers showed that 2 percent of women
were starting their fertile window by day four, 17 percent by day
seven. More than 70 percent of normal women were in their fertile
window before day 10 or after day 17, leaving few universal "safe"
days for such natural birth control systems as the "rhythm method."
Even women who regard their cycles as "regular" had a 1 to 6 percent
probability of being fertile on the day their next period was
To obtain precise fertility information, the study required
that the women volunteers provide daily urine samples that the
researchers could test for estrogen and progesterone. An abrupt
change in the ratio of these hormones marks the event of ovulation.
This signal was used to identify days of ovulation during 696
In previous reports, Wilcox and colleagues had shown that the
fertile window spans six days, ending with the day of ovulation. In
the new report they find that sporadic late ovulation occurs even in
women who say their cycles are usually regular. Because of these late
ovulations, the researchers show, 4 to 6 percent of women are
potentially fertile in the fifth week more than 28 days after the
start of their cycle.
Having intercourse during a fertile day does not always
produce a pregnancy, which also depends on the viability of the sperm
and egg, the receptivity of the uterus and other factors that vary
widely among couples.
NIEHS, which is one of the National Institutes of Health, has
its laboratories in Research Triangle Park, N.C., near Raleigh, Durham
and Chapel Hill. The test subjects were women living in this area.
The NIEHS researchers said their report describes the only
modern test data they are aware of on the range of the fertile window.
However, they note that during World War I, a German physician
described 25 pregnancies reportedly produced by single acts of
intercourse during soldiers' leaves. The reported acts of isolated
intercourse occurred from Cycle Day Two to Cycle Day 30 according to
the memories and circumstances told to the doctor. Now, 84 years
later, this new study suggests that doctor's report was entirely
Dr. Wilcox is available at (919) 541-4660.