Looking For That Fertile Window?

Some Research Says Women Ovulate All Over the Calendar

If you're trying to conceive, you've heard of the "fertile window," and you're probably focusing on your own right now. For the uninitiated, the term refers to a reportedly very small period of time during a woman's cycle when she is able to get pregnant.

Now comes down the pike a research study that may leave our sympto-thermal-charting friends' heads spinning.

The Big Story

A study published in an issue of the esteemed British Medical Journal says throw away the calendar and look skeptically at gauging your fertile time by gazing at cervical mucus and thermometers, even if you're a woman of normal fertility.

In "The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study" (Wilcox A, Dunson D, Baird D. BMJ 2000;321:1259-1262), researchers conclude 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

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 35 years)
  • women with no known fertility problems
  • women who were seeking pregnancy
The majority of volunteers were college educated, white, and two-thirds had never conceived. Fertility was gauged by daily measuring levels 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 whether 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.