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.