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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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Is LMP obsolete in obstetrics and gynaecology?

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

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Competing interests: No competing interests

14 December 2000
Angus JM Thomson
Specialist Registrar, Obstetrics and Gynaecology
Liverpool Women's Hospital, UK.