This Week In The Bmj

Fertile days are unpredictable

BMJ 2000; 321 doi: http://dx.doi.org/10.1136/bmj.321.7271.0/e (Published 18 November 2000) Cite this as: BMJ 2000;321:e

Guidelines for getting pregnant, or avoiding pregnancy, usually assume that the average woman is fertile between days 10 and 17 of her menstrual cycle. On p 1259, Wilcox et al show that fertile days can fall almost any time during the menstrual cycle—even among women with “regular” cycles. This is due to the natural variability of ovulation, described by the authors in a cohort of 213 healthy women attempting to conceive. Women had a 1-6% probability of being fecund, even on the day their next period was expected.