Intended for healthcare professionals

Rapid response to:


The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study

BMJ 2000; 321 doi: (Published 18 November 2000) Cite this as: BMJ 2000;321:1259

Rapid Response:

Evidence-based fertility awareness methods

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

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

Jane Knight

Fertility Nurse Researcher, Bury Knowle Health Centre,
207 London Road,

& Director Fertility UK, Clitherow House, 1 Blythe Mews, London W14

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

04 January 2001
Jane Knight