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:

NFP does not rely on prediction

The paper by Wilcox et al on the timing of the "fertile window" as
well as some of the responses of Wilcox to different experts in Natural
Family Planning (NFP) are unfortunate for several reasons:

1) The results from Wilcox are not extremely useful nor are they very
relevant for NFP as we use them today. The bottom line is, Wilcox and
coinvestigators seem unaware of how really NFP works today.

2) NFP does not rely on prediction. We do not really need to predict
ovulation six days before it occurs for NFP to work and to be useful. The
issue is whether women are able to identify fertility indicators early
enough to avoid a pregnancy if this is their choice or to increase the
probablity of a pregnancy if they conversely want a pregnancy. And
evidenced based medicine exists on this topic.

3) Wilcox has performed a prospective study using 221 healthy women.
This is fine but they should have mentioned another prospective cohort
study published in 1999 in Advances in Contraception (European multicenter
study of natural family planning (1989-1995): efficacy and drop out. Adv
in Contr 1999;15:69-83). This too was a prospective study although with
the analysis of data on 1260 women and 18360 cycles using the
symptothermal NFP method where an unintended pregnancy rate of 2.6% was
reported (this rate included user related failures). How can these results
be possible if the prediction of ovulation is as Wilcox says "highly
unpredictable"?....maybe the response is that we do not need to predict
ovulation to properly use NFP today. This should have been included in
their discussion.

4) I should remind the responders that have included words about the
Vatican that this is a scientific journal and not a pamphlet. If you have
nothing scientific to say please no insults and refrain from wasting
cyber space.

Jokin de Irala

Competing interests: No competing interests

12 January 2001
Jokin de Irala
Professor of Epidemiology and Public Health
Medical School, University of Navarre