Intended for healthcare professionals

Rapid response to:


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

Rapid Response:

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

Dear Sirs - We are writing in response to the November 16th
publication in the BMJ of a paper by Wilcox et al ("The timing of the
fertile window in the menstrual cycle: day specific estimates from a
prospective study"). Their findings support many of the results of our
own work, although there are some differences which are probably explained
by differences in our study populations.

We (at the Georgetown University Medical Center, Institute for
Reproductive Health) are carrying out a prospective multi-center efficacy
trial for a family planning method recommending that women abstain from
unprotected intercourse on days 8 through 19 of every cycle to avoid
pregnancy. (An article describing the development of this formula was
published in Contraception, 1999;60:357-360, by Arevalo, Sinai and
Jennings.) Our study population is 550 women in Bolivia, Peru and the
Philippines, who are being followed for 13 cycles. We are providing the
method only to women with cycles between 26 and 32 days long, screening
out women with shorter or longer cycles precisely because they would tend
to be fertile earlier or later than days 8 through 19 and thus would be
placed at undue risk of pregnancy. Furthermore, the method considers a
total of 12 days as potentially fertile, taking into account the 6-day
fertile window plus the potential variability in the timing of ovulation.

Preliminary results of this ongoing trial confirm that the method provides
sufficient protection from unplanned pregnancy.
With regard to our respective study populations, Wilcox et al. included
women who reported habitual cycle length as short as 19 days and as long
as 60 days. As noted, we screen out women with cycles shorter than 26
days or longer than 32 days. Some percentage of women in the Wilcox study
were using other family planning methods, some of them hormonal methods,
shortly before entering the study. We screen out women who have used
hormonal methods during the preceding three months.
We expect that the results of our efficacy trial, which will be available
in mid-2001, will further confirm Wilcox' findings.

Victoria H. Jennings, Ph.D.

Professor, Principal Investigator

Marcos Arevalo, M.D., MPH

Director, Field Trials

Institute for Reproductive Health, Georgetown University Medical
3800 Reservoir Road, NW - 3PHC, Washington, D.C. 20007 USA

Competing interests: No competing interests

04 January 2001
Victoria H Jennings