Wilcox, A. J., Dunson, D., & Baird, D. D. (2000). The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ (Clinical Research Ed.), 321(7271), 1259-1262. https://doi.org/10.1136/bmj.321.7271.1259
Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-1262. doi:10.1136/bmj.321.7271.1259
Wilcox, A. J., et al. "The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study." BMJ (Clinical Research Ed.), vol. 321, no. 7271, 2000, pp. 1259-1262.
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RRM Academy Synopsis
The fertile window falls on widely different cycle days
In a prospective cohort of 221 women planning a pregnancy, the fertile window landed on widely varying cycle days. Only about 30% of women had all six fertile days between days 10 and 17. Daily urine hormone tests gave ovulation timing for 696 cycles.
Key Findings
From day 6 through day 21, every cycle day carried at least a 10% probability of falling in a woman's fertile window.
An estimated 2% of women were fertile by cycle day 4 and 17% by day 7. The share peaked at 54% on days 12 and 13.
The earliest ovulation came on day 8 and the latest on day 60. In the fifth week, 4-6% of women still waiting for menses were in their fertile window.
Among 69 cycles of 28 days, only 10% had ovulation exactly 14 days ahead of the following menses.
An estimated one third of 39 women with short usual cycles had reached their fertile window in the first week, against 7% of 55 women with long usual cycles.
Interpretation
This cohort study tracked women over time. Each day, urine hormone tests marked the ovulation day of each cycle. Women with irregular cycles tended to ovulate later, on more varied days. The authors note that no single marker matches egg release perfectly. The findings support larger studies of fertile-window timing in more groups of women.
RRM Context
Fertility awareness-based methods track the signs of a woman's own cycle, such as cervical mucus. This study shows why that matters. Ovulation timing varies a lot, both between women and between cycles.
Our editorial summary of this paper, not the article's abstract.
Abstract
Objectives
To provide specific estimates of the likely occurrence of the six fertile days (the "fertile window") during the menstrual cycle.
Design
Prospective cohort study.
Participants
221 healthy women who were planning a pregnancy.
Main Outcome Measures
The timing of ovulation in 696 menstrual cycles, estimated using urinary metabolites of oestrogen and progesterone.
Results
The fertile window occurred during a broad range of days in the menstrual cycle. On every day between days 6 and 21, women had at minimum a 10% probability of being in their fertile window. Women cannot predict a sporadic late ovulation; 4-6% of women whose cycles had not yet resumed were potentially fertile in the fifth week of their cycle.
Conclusions
In only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines-that is, between days 10 and 17. Most women reach their fertile window earlier and others much later. Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular.
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