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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: https://doi.org/10.1136/bmj.321.7271.1259 (Published 18 November 2000) Cite this as: BMJ 2000;321:1259

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The author comments

Thanks to those who have responded to our paper. I'd like to address
just a couple of points.

We did not collect information on the signs or symptoms relevant to
NFP. The purpose of our paper was to describe the variability of ovulation
day in the menstrual cycle. Ovulation is of course the benchmark for
describing when a woman is fertile in her cycle. We found that the cycle
day of ovulation (and hence the fertile days preceding ovulation) varied
widely.

We know from previous reports that women are fertile (on average) for
six days in each cycle ending with the day of ovulation. To my knowledge,
there is no reliable way to predict ovulation six days in advance. (If
anyone has information on this point, I'd be very interested in seeing
it.) The unpredictability of ovulation consequently makes it difficult to
know when the earliest fertile days occur in the cycle. Our data suggest
that relying on day of the cycle alone (not what NFP does, but apparently
a method of fertility regulation for many couples) is unreliable. The
unreliability of the calendar method is of course well known. Our paper
adds specific data on this point.

Going beyond our data, I'm not aware that the more sophisticated
methods of NFP (monitoring of cervical mucus, BBT or any of the commercial
lab kits) are able to predict ovulation accurately six days ahead of time.
The gold standard for ovulation in such a study would have to be
ultrasound or LH surge (for example, see Guida et al, Fertility &
Sterility 72:900-4, 1999). Any method that could be shown to predict
ovulation by six days would be enormously useful. As it stands, the
reliability of any methods for predicting ovulation more than a day or two
ahead (including the most sophisticated laboratory kits) appears poor.

Of course, this is not to say that some couples cannot have success
with NFP, if they are willing to observe a long enough window of
abstinence. The question here is more fundamental -- can we dependably and
accurately predict ovulation six days before it occurs? If this could be
demonstrated, it would provide the basis for more efficient NFP.

Competing interests: No competing interests

27 November 2000
Allen Wilcox
Chief, Epidemiology Branch
NIEHS/NIH, USA