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;321:1259All rapid responses
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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.
Sincerely,
Jokin de Irala
Competing interests: No competing interests
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
Center,
3800 Reservoir Road, NW - 3PHC, Washington, D.C. 20007 USA
Competing interests: No competing interests
Editor, The well designed paper from Wilcox et al raises some
interesting dilemmas about the timing of fitting post-coital IUCDs.
Standard advice is that an IUCD may be fitted 'in good faith' up to five
days after the calculated earliest date of ovulation. (Faculty of Family
Planning and Reproductive Healthcare 2000) Until now calculation of this
date has been based on the assumption that ovulation occurs exactly 14
days before menstruation. If it may in fact be up to 19 days before
menstruation, there is a risk of fitting an IUCD after implantation has
occurred. Perhaps the guidance needs to be reviewed in the light of this
paper?
No competing interests.
Ref: Guidance April 2000. Emergency contraception: recommendations
for clinical practice. BJFP 26(2);Apr 2000:93-96
Competing interests: No competing interests
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
20(17+3)."
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
study.
Allen J Wilcox, David Dunson, Donna Day Baird.
Competing interests: No competing interests
EDITOR - The latest research from Wilcox, Dunson and Baird confirms
yet again the unreliability of calendar calculations for most women.
Modern fertility awareness methods are evidence-based and do not rely on
calculations alone. They combine the indicators of fertility
(temperature, cervical secretions, changes in the cervix and a shortest
cycle calculation) to give the most accurate interpretation of the fertile
time. These subjective signs of fertility have been correlated with
hormone profiles and ultrasound for many years. Increasing numbers of
women in UK also choose to correlate these subjective indicators with the
more objective marker Persona - the personal hormone monitoring system.
This combined approach provides the most accurate information about the
limits of the fertile time and allows a well-informed user to make
responsible decisions about modifying sexual behaviour to accommodate
abstinence or the use of barrier methods during the fertile time. The
effectiveness of using a combination of indicators is clearly established
as up to 98% effective using prospective multi-centred studies. (Freundl
1999).
For most couples planning pregnancy - the guideline days 10-17 are
also likely to be the least helpful. The key indicator for timing
intercourse to conceive is the recognition of fertile secretions (wetter,
clear, slippery mucus secretions) which maintains the life of the sperm -
basic information, but a sign which can be recognised by around 70% women.
The value of this signal was recognised by the Wilcox team in an earlier
paper: 'Cervical mucus change provides an earlier and more useful cue.
(compared with basal body temperature or urinary LH kits). Mucus
receptivity begins several days before ovulation so couples who have
frequent intercourse after this cue will tend to have intercourse on those
days with the highest probabilities of clinical pregnancy.'
In UK, education in fertility awareness methods is increasingly available
through trained health professionals in Primary Care. Multi-disciplinary
health professional training is available through the Fertility UK course
(University of Greenwich credit-rated course). The main UK evidence-based
web site providing information on fertility awareness methods can be found
at www.fertilityuk.org.
Jane Knight
Fertility Nurse Researcher, Bury Knowle Health Centre,
207 London Road,
Oxford.
& Director Fertility UK, Clitherow House, 1 Blythe Mews, London W14
0NW
1. Wilcox A. J., Dunson D, Baird DD., The timing of the 'fertile
window' in the menstrual cycle: day specific estimates from a prospective
study. BMJ Vol 321; 18 November 2000. 1259-1262.
2. Flynn A, Docker M, Morris R, Lynch S Royston J. 'The reliability
of women's subjective assessment of the fertile period, relative to
urinary gonadotrophins and follicular ultrasonic measurements during the
menstrual cycle', In Bonnar J, Thompson W, Harrison RF, eds. Research in
Family Planning, Lancaster, England, 1983; 3-11.
3. Bonnar, J., Flynn A., Freundl G., Kirkman R., Royston R. &
Snowdon R. Personal hormone monitoring for contraception. The British
Journal of family planning. 1999; 24: 128-134.
4. Freundl, G.,. European multi-centre study of natural family
planning (1989-1995). Advances in contraception. 1999; 15: 69-83
5. Dunson DB, Baird DD, Wilcox AJ, Weinberg CR, Day specific
probabilities of clinical pregnancy based on two studies with imperfect
measures of ovulation. Human Reprod 1999, Jul; 14 (7): 1835-9
Competing interests: No competing interests
EDITOR – Doctors have historically been taught that the last
menstrual period (LMP) is an essential part of history taking required to
guide management in both obstetric and gynaecological cases. More
recently, as investigations such as beta-hCG and ultrasound improve,
the role of LMP in hospital management is ever decreasing. The paper by
Wilcox et al.1 is yet another nail in the coffin of the LMP.
In their paper Wilcox et al. dispel the popular myth that ovulation
always occurs on day 14 of a regular 28-day cycle. They have convincingly
illustrated that the ‘fertile window’ can vary enormously and that the
majority (70%) of women has reached their fertile period before the 10th
day of their cycle. No longer can we confidently inform couples in the
infertility clinic when their most fertile period has arrived based on LMP
and cycle length alone. Many of these patients are already using urinary
ovulation prediction kits to give themselves greater chance of success.
The data in this article corroborates our own study. Of 106 women
reporting a regular 28 day cycle, only 26 (25%) had urinary LH surge on
day 14 as would traditionally be expected.
Not only is LMP an inaccurate method of calculating the fertile
window but also, once pregnant, LMP is now of increasingly little use to
the clinician. Ectopic pregnancy is no longer diagnosed by laparoscopy
when appropriate signs are present after a certain length of amenorrhoea.
Instead ectopics are now diagnosed by transvaginal ultrasound and
beta-hCG measurement, as Ankum informs us in his editorial in the
same edition of the BMJ2. Furthermore, many units now calculate gestation
of pregnancy by ultrasound (crown rump length in the first trimester or
head circumference at 20 weeks gestation) rather than from the LMP as this
is shown to be more accurate3. Ultrasound estimation of gestation also
reduces the incidence of inductions of labour for post-maturity4.
We would not wish to rely on investigations alone or to lose history
taking skills, however the clinical usefulness of LMP is certainly
declining.
Angus Thomson – Specialist Registrar in Obstetrics and Gynaecology
Liverpool Women’s Hospital, Crown Street, Liverpool. L8 7SS
E-mail: gus@doctors.org.uk
Andrew Drakeley – Specialist Registrar in Obstetrics and Gynaecology
Liverpool Women’s Hospital, Crown Street, Liverpool. L8 7SS
Charles Kingsland – Consultant Obstetrician and Gynaecologist
Liverpool Women’s Hospital, Crown Street, Liverpool. L8 7SS
1. Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window”
in the menstrual cycle: day specific estimates from a prospective study.
BMJ 2000;321:1259-62
2. Ankum WM. Diagnosing suspected ectopic pregnancy. BMJ 2000;321:1235-36
3. Montgelli M, Wilcox M, Gardosi J. Estimating the date of confinement:
Ultrasonographic biometry versus certain menstrual dates. Am J Obstet
Gynecol;13:103-106
4. Bersjø P, Denman DW, Hoffman HJ, Meirik O. Duration of human singleton
pregnancy – a population-based study. Acta Obstet Gynecol Scand
1990;69:197-207
No Conflict of Interest
Corresponding author – Angus Thomson
YOU RECEIVED THE E-MAILS?)
Competing interests: No competing interests
The well-designed study by Wilcox, Dunson and Baird clearly
demonstrates the variability of the ovulation day in the menstrual cycle.
This information might be of significant relevance for couples trying to
conceive, especially where the male partner has some degree of impaired
fertility, e.g. because of diminished progressive sperm motility
(Nieschlag & Behre, 2000). Optimisation of the timing between sexual
intercourse and ovulation could enhance chances of achieving a pregnancy.
In their introduction Wilcox et al. say that “Reliable methods to
predict ovulation are lacking, therefore predicting the fertile window is
also unreliable.” The method of daily urine sampling and assaying of
hormone concentrations to detect ovulation used in the study by Wilcox et
al. is not suitable for home use. In the December issue of Human
Reproduction we published the results of a prospective study on the
performance of the ClearPlan® Fertility Monitor (CPFM, Unipath Ltd.,
Bedford, U.K.) to predict and detect ovulation (Behre et al. 2000). The
CPFM comprises a hand-held monitor and disposable dual-assay urine test
sticks, which simultaneously detect LH and estrone-3-glucuronide
concentrations in early morning urine. In our study involving 53 women for
up to four consecutive menstrual cycles, ovulation was detected in 91.1%
of cycles during the 2 days of CPFM peak fertility display on the monitor,
and - of similar importance - never occurred before. Home use of CPFM
could help couples who desire pregnancy to time intercourse and may be
applied in the initial treatment of infertility.
References:
Behre HM, Kuhlage J, Gassner C, Sonntag B, Schem C, Schneider HPG,
Nieschlag E. Prediction of ovulation by urinary hormone measurements with
the home use ClearPlan® Fertility Monitor: comparison with transvaginal
ultrasound scans and serum hormone measurements. Human Reproduction 2000
Dec;15:2478-2482
Nieschlag E, Behre HM. Andrology – Male Reproductive Health and
Dysfunction. 2nd edition. Springer, 2000.
Competing interests: No competing interests
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
Some concern have been raised about the generalizability of our
results due to the fact that women had discontinued their use of any birth
control just prior to enrollment. To the extent that prior birth control
methods were hormonal, there could be residual effects on patterns of
ovulation during the period of observation.
In fact, only 5% of women were using oral contraceptives just prior
to entering the study, and none had been using injectables. (These data
have been provided in detail in earlier papers, e.g. NEJM 319:189-94,
1988.) The three most common methods of birth control just prior to
enrollment were the diaphragm (40%), condom (29%) and NFP (13%).
Thus, we find no reason to believe that the variable ovulation seen
in our study could be an artifact of prior birth control use.
Regarding the exclusion of couples with known fertility problems,
this was to avoid the over-representation of such couples. This is not to
say that all women in our study were fertile. To the contrary, about ten
percent had not conceived within their first year of trying -- similar to
estimates from the general population.
Competing interests: No competing interests
The luteal phase may be more constant than you think
The authors conclude that the length of the luteal phase is highly
variable (and therefore cannot be used in family planning). How does this
fit with the work by Lenton et al 1984 (Br J Obstet Gynaecol, 1984,
91[7]:685-9), which showed for a large sample of ovulating women that the
length of the luteal phase is normally-distributed with a mean of 14.13 +/
- 1.41 days? Lenton et al's result suggests that the
luteal phase is constant to within a couple of days, not only for a given
woman, but amongst women in general. (The authors noted a tail, comprising
5% of the sample, of short luteal phases that they associated with
abnormal cycles.)
I would like to have seen in Wilcox et al's paper a plot showing
length of luteal phase versus length of cycle. If the authors are correct,
this plot will be a scatter diagram. If on the other hand Lenton et al are
right, the data points will fall in a narrow, horizontal band centred on
14.1 days. Disregarding the abnormal cycles, 68% of the points will lie
within 12.7 and 15.5 days, and 95% within 11.3 and 16.9 days.
This range is adequately covered by the guidelines for natural family
planning, which tell couples to avoid intercourse for +/- 5 days around
their predicted ovulation date, i.e., the expected date of the woman's
next period minus 14 days. (In fact most users employ the much more
reliable temperature and mucus indicators in addition to the date
estimator.)
Unless I missed something, I also didn't see a discussion of the
uncertainty intrinsic to the method Wilcox et al used to determine the
day of ovulation.
Competing interests: No competing interests