New Research Says Women Ovulate All Over the Calendar
Women who are trying to conceive may have heard of the "fertile
window". For those who haven’t, the term refers to a reportedly
very small period of time during a woman's cycle when she is able
to get pregnant.
A study published in the British Medical Journal titled:
"The timing of the "fertile window" in the menstrual
cycle by Wilcox A, Dunson D, Baird D. concluded that only about 30%
of women actually experienced the typically referred-to "fertile
window" (between days 10 and 17 of a cycle), even when they
have reported regular cycles.
According to Dr. Allen J. Wilcox, Chief of Epidemiology Branch of
the National Institute of Environmental Health Sciences (NIEHS), "The
study was not designed with infertile women in mind, but it strongly
implies that, on average, a woman's typical cycle length is very
important to her ability to conceive."
The study, which took place in Durham, North Carolina where NIEHS
is located, looked at:
- women in their typically prime reproductive years (age 25 to
- women with no known fertility problems
- women who were seeking
The majority of volunteers were college educated, white,
and two-thirds had never conceived. Fertility was gauged by daily
of estrogen and progesterone in the participants' urine.
In the statistical analyses, day-specific probability of fertility
was calculated for
the entire group, for women were subgrouped according to
or not they reported regular or irregular cycles, and for
regular-cycled women by their usual cycle length.
Dr. Wilcox noted that participants were not selected based on the
regularity of ovulation cycle, but on their history of fertility
issues or lack thereof.
Of 696 cycles observed in 213 women:
- 2% were fertile by cycle day 4
- 17% were fertile by cycle day
- 54% were fertile by cycle day 12 or 13
- women who reported usual
cycles of 27 days or less ovulated, and therefore were fertile,
earlier than those with cycles
- estimated one third of women with short cycles
reached fertility by end of cycle week one
- calculated a 1-6%
probability of being fertile on the day the next period was expected
- greater than 70% are fertile either before cycle day 10 or after
cycle day 17
- some women are potentially fertile during most of
the days in their cycle
It is important to note that the cycle days usually described
as the fertile window are still considered to be the five days
prior to ovulation and the day of ovulation itself. This study found
evidence supporting current medical opinion that ovulation prior
to cycle day 13 indicates a less fertile cycle. One participant's
ovulation on cycle day 8 resulted in a successful pregnancy.
The researchers sought to update assumptions about women's most
fertile times, and that, they have. Current assumption is that, on
average, women ovulate 14 days prior to next menstrual period. In
this study of 213 women, out of 69 28-day (thought to be the norm)
cycles, ovulation occurred 14 days prior to menses in only ten percent.
Ovulation was noted to occur from cycle day 10 to 22, and at least
10% of regular-cycled women were fertile on any given day between
cycle days 6 and 21. The outer ends of the fertility spectrum, adolescence
and perimenopause, provide even less predictability.
What Does It All Mean?
Aside from confirmation that some women really are as fertile as
ever, and validation that the calendar method of predicting fertility
is fruitless, the results of this study mean that, for couples who
are trying to conceive, knowing when a woman ovulates is important.