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The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study

BMJ 2000; 321 doi: (Published 18 November 2000) Cite this as: BMJ 2000;321:1259
  1. Allen J Wilcox (wilcox{at}, branch chiefa,
  2. David Dunson, investigatorb,
  3. Donna Day Baird, senior investigatora
  1. a Epidemiology Branch, National Institute of Environmental Health Sciences, Durham, NC 27709, USA
  2. b Biostatistics Branch, National Institute of Environmental Health Sciences
  1. Correspondence to: A J Wilcox
  • Accepted 30 August 2000


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.


  • Funding This project was conducted as part of the intramural research programme of the National Institute of Environmental Health Sciences, National Institutes of Health.

  • Competing interests None declared.

  • Accepted 30 August 2000
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