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. 2000 Nov 18;321(7271):1259-62.
doi: 10.1136/bmj.321.7271.1259.

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

A J Wilcox  1 , D Dunson, D D Baird
Affiliations

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

A J Wilcox et al. BMJ. .

Abstract

Objectives: To provide specific estimates of the likely occurrence of the six fertile days (the "fertile window") during the menstrual cycle.

Design: Prospective cohort study.

Participants: 221 healthy women who were planning a pregnancy.

Main outcome measures: The timing of ovulation in 696 menstrual cycles, estimated using urinary metabolites of oestrogen and progesterone.

Results: The fertile window occurred during a broad range of days in the menstrual cycle. On every day between days 6 and 21, women had at minimum a 10% probability of being in their fertile window. Women cannot predict a sporadic late ovulation; 4-6% of women whose cycles had not yet resumed were potentially fertile in the fifth week of their cycle.

Conclusions: In only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines-that is, between days 10 and 17. Most women reach their fertile window earlier and others much later. Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular.

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Figures

Figure 1
Figure 1
Probability of being in fertile window by day of menstrual cycle
Figure 2
Figure 2
Probability of women with regular or irregular cycles being in their fertile window
Figure 3
Figure 3
Probability of women with regular cycles being in fertile window, stratified by usual cycle length
See this image and copyright information in PMC

Comment in

References

    1. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy and sex of the baby. N Engl J Med. 1995;333:517–521. - PubMed
    1. Wilcox AJ, Weinberg CR, Baird DD. Post-ovulatory ageing of the human oocyte and embryo failure. Hum Reprod. 1998;13:394–397. - PubMed
    1. Dunson DB, Baird DD, Wilcox AJ, Weinberg CR. Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation. Hum Reprod. 1999;14:1835–1839. - PubMed
    1. Wilcox AJ, Weinberg CR, O'Connor JF, Baird DD, Schlatterer JP, Canfield RE, et al. Incidence of early loss of pregnancy. N Engl J Med. 1988;319:189–194. - PubMed
    1. Wilcox AJ, Weinberg CR, Wehmann RE, Armstrong EG, Canfield RE, Nisula BC. Measuring early pregnancy loss: laboratory and field methods. Fertil Steril. 1985;44:366–374. - PubMed

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