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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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recent contraception limits relevance of findings

Dear Sir,

Wilcox and colleagues have revisited the topic of the regularity (or
otherwise) of ovulation to aid with the advice of women either trying to
conceive or avoid conception (1). The data presented shows a wide curve
predicting the chance of fertility at any given time in the menstrual
cycle. The fact that women were recruited at the time of discontinuation
of their chosen method of birth control throws numerous doubts on their
findings.

It has been known for some time that fertility is affected following
discontinuation of both oral and injectable contraceptives (2,3). Some
injectable contraceptives however allow fertility to return to normal
rapidly after discontinuation (4). If this decreased fertility is via
disruption of ovulation then it is of no great surprise that the date of
ovulation varies so greatly and the curve of probability of fertility is
so wide in this group. The methods of contraception are not stated.

It is also stated that women with known fertility problems were
excluded from the group, although the method of diagnosis is not clear,
especially as these women used contraception until the point of entry to
the study.

At enrollment into the study information was collected on menstrual
history, this data is of course questionable if it pertains to the period
of time using hormonal contraceptives and is not a reflection of the
woman’s natural menstrual cycle.

This study is of help in the counselling of women who have recently
stopped use of contraceptives, although which contraceptives exactly is
not clear from this paper. There are difficulties applying these findings
to the general population.

Yours sincerely,

Dr JCR Hardwick
MRCOG


Specialist Registrar Obstetrics and Gynaecology

Glasgow Royal Maternity Hospital,
Rottenrow,
Glasgow
G4 0NA

References.

1. Wilcox AJ, Dunson D, Day Baird D. The timing of the ‘fertile
window’ in the menstrual cycle: day specific estimates from a prospective
study. BMJ 2000;321:1259-63

2. Bracken MB. Hellenbrand KG. Holford TR. Conception delay after oral
contraceptive use: the effect of estrogen dose. Fertility & Sterility.
1990; 53:21-7, 1990

3. Weisberg E. Fertility after discontinuation of oral contraceptives.
Clinical Reproduction & Fertility 1982;1:261-72

4. Bahamondes L. Lavin P. Ojeda G. Petta C. Diaz J. Maradiegue E. Monteiro
I. Return of fertility after discontinuation of the once-a-month
injectable contraceptive Cyclofem. Contraception. 1997;55:307-10

Competing interests: No competing interests

28 November 2000
J C R Hardwick
specialist registrar obstetrics and gynaecology
glasgow royal maternity hospital