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 the following:

  • 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 sub-grouped 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.

The Study Results

Of 696 cycles observed in 213 women that

  • 2% were fertile by cycle day 4
  • 17% were fertile by cycle day 7
  • 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 longer than 27 days
  • 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 no evidence supporting the 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 they have succeeded. 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 much more fertile than ever before believed, and validation that the calendar method of predicting fertility is fruitless, the results of this study mean that, for some couples who are trying to conceive, timing intercourse precisely need not be the focus it has been. Again note: this study excluded any women with known fertility problems. However, for those who are of average fertility (and all other factors leading to successful conception are in place and functioning), these researchers suggest that conception should occur through sexual intercourse twice or more times a week–without even looking at the calendar.

On one of this site’s favorite topics, using sympto-thermal methods or home kits to predict ovulation, Dr. Wilcox concludes, “(Those methods) are fine for detecting the day of ovulation, but still leave much to be desired in predicting the prior fertile window.”