Retrospective vs. prospective coital frequency and menstrual cycle length in a contraceptive effectiveness trial

PURPOSE: To determine how well information at enrollment would predict coit al frequency and menstrual segment length during a prospective contraceptiv e effectiveness trial. METHODS: We compared retrospective reports of monthly coital frequency and menstrual segment (cycle) length with prospective information for women par ticipating in a contraceptive trial of the Reality((R)) female condom. RESULTS: Participants reported slightly higher mean monthly coital frequenc y and slightly longer menstrual segments prior to the study than during the study ( 12.6 acts vs. 12.0 acts per month and 30.8 days vs. 28,4 days). We examined the actual distribution of differences between the retrospective and prospective measures and found considerable variability. Among the 195 participants studied, parous women were less likely to predict accurately m enstrual segment length (OR 0.4; 95% Cl 0.3-0.9), while older women were mo re likely to predict accurately coital frequency (OR 1.9; 95% CI 0.9-3.3). CONCLUSIONS: Coital frequency and menstrual segment length vary considerabl y over time. Hence, prospective data collection is essential to accurately characterize these variations and to property interpret results from contra ceptive trials and other studies concerned with fecundability and hazards o f reproduction. (C) 2001, Elsevier Science Inc. Alt rights reserved.