s reports of the usual
length of their
cycle. The women reported usual
cycle lengths as short as 19 days and as long as 60 days, with 28 days being the most common. We found substantial correlation between usual cycle length at enrolment and day of ovulation (follicular phase length) during the study (R=0.55, all cycles). Thus, self reported cycle length can be useful in predicting whether a woman is in her fertile window.