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

Journal Article (Journal Article)


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


Prospective cohort study.


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.


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.


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.

Full Text

Duke Authors

Cited Authors

  • Wilcox, AJ; Dunson, D; Baird, DD

Published Date

  • November 2000

Published In

Volume / Issue

  • 321 / 7271

Start / End Page

  • 1259 - 1262

PubMed ID

  • 11082086

Pubmed Central ID

  • PMC27529

Electronic International Standard Serial Number (EISSN)

  • 1756-1833

International Standard Serial Number (ISSN)

  • 0959-8138

Digital Object Identifier (DOI)

  • 10.1136/bmj.321.7271.1259


  • eng