BMJ Allen J Wilcox , David Dunson and Donna Day Baird study cycle : day specific estimates from a prospective

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. What is already known on this topic? Both short and long intervals between pregnancies are associated with adverse perinatal outcomes Conflicting data exist on the impact of interpregnancy interval on maternal morbidity and mortality What does this study add? Women with interpregnancy intervals shorter than 6 months are at increased risk of maternal death, third trimester bleeding, premature rupture of membranes, puerperal endometritis, and anaemia Women with interpregnancy intervals longer than 59 months are at increased risk of pre-eclampsia and eclampsia As the research was carried out in developing countries the results may not apply to other populations Papers Epidemiology Branch, National Institute of Environmental Health Sciences, Durham, NC 27709, USA Allen J Wilcox branch chief Donna Day Baird senior investigator