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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

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The timing of the "fertile window" in the menstrual cycle

In the BMJ article Wilcox quotes selectively from
Beckmann's guidance for women using the calendar
method of natural family planning, (-a family
planning method no longer promoted in UK). He
therefore does his modeling on a woman having
completely regular 28-day cycles. In fact the
guidance states: "When the calendar is used, the
fertile period would last from days 10 through 17
for a woman with an absolutely regular 28-day
cycle. Additional days are added to the fertile
period based on time of shortest and longest
menstrual interval. Thus a woman with periods
every 28 plus or minus 3 days would be
considered to be fertile from days 7 (10-3) to

It would have been more realistic for
Wilcox et al, to use 28 days plus or minus3 days
for this modeling. The Institute for Reproductive
Health, Georgetown University have clearly have
taken this approach using a blanket rule that states
for women with menstrual cycle lengths between 26
to 32 days are likely to be fertile on days 8 to 19
inclusive. If this more realistic approach had been
used then the coverage of the fertile days by the
calendar period would be more accurate.

Dr Cecilia Pyper

Primary Care Career Scientist

Health Services Research Unit,
Department of Public Health,
Institute of Health Sciences,
University of Oxford,
Old Road,
Oxford OX3 7LF

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

Allen J Wilcox, David Dunson, Donna Day Baird.

Competing interests: No competing interests

04 January 2001
Cecilia Pyper