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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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Location of fertile window determines length of the cycle - known for decades by BOM usersngth of

So, Dr. Wilcox’s team has found that a woman’s window of fertility
can occur at any time in her cycle and that therefore, the
“calendar/rhythm/ temperature taking” package can finally be laid to rest
as effective, natural ways of postponing pregnancy.  Hurrah!  Well this
may be a great discovery for Dr. Wilcox but certainly NOT to those who
have been using the Billings Ovulation Method of modern natural family
planning for many years. They know that the “window of fertility” is
unpredictable, but that it is recognisable when it starts.

To exploit knowing when she is fertile, either to achieve or avoid
pregnancy, a woman only has to recognise the start of her fertile phase,
not predict when it will occur. If Dr. Wilcox were to consult the website
www.billingsmethod.com he would learn that a woman can recognize the
first day of her fertile window by the appearance and familiar sensation
of a mucus secretion, although she cannot predict when it will occur. It
may not occur in many of her cycles; stress, pre-menopause, breast-
feeding, coming off contraceptive medication are all factors that delay
ovulation and common sense guidelines provide the couple with confidence
during these prolonged pre-ovulatory days. In effect, there is no such
thing as “regular” cycles. The Billings doctors called their method the
“ovulation method” because it is ovulation that sets the length of the
cycle. Fluctuating hormones controlling the length of the pre-ovulatory
phase can continue for many months causing possible spotting and patches
of mucus. When they finally get their act together to rise to the required
level in one day (Peak) ovulation is triggered a fraction of a day later,
causing menstruation to follow a constant two weeks later (11 - 16 days).

If the couple wait during the few varying numbering days of mucus
that mark the "window" until the fourth day after the abrupt change in
sensation (Peak Day) they can enjoy intercourse for the next two weeks (11
- 16 days) without becoming pregnant, if that is their intention.

By keeping a simple nightly record the couple, whether Catholic,
Hindu or Evangelical, knows when intercourse could possibly lead to
pregnancy and when it cannot possibly do so with 99% accuracy.  (W.H.O.
five nation trial, 1978). If the intention is to conceive they can use
this information to time intercourse on, in rare cases, the one day in the
year when the mucus, that is so vital for sperm survival and transport,
is present.  Moreover, the woman WILL be able to predict when she should
menstruate two weeks ahead of time  If she does not menstruate because
pregnancy has been achieved, she will be able to tell her doctor when the
baby is due (266 days plus or minus 6 days from her last recorded Peak
Day). Her chart will also provide a diagnostic tool for him when there is
a radical departure from her normal pattern of fertility and infertility.

There is much that has been known to women for ages that Dr. Wilcox
has yet to “discover.”

Susan and Justin Fryer

Competing interests: No competing interests

03 December 2000
Justin Fryer
President Calgary Billings Centre
Susan Fryer
Colonel Belcher hospital