This study proposes a behavioral model that identifies determinants of coital activity in the context of pregnancy avoidance and assesses the relationships using weekly panel data collected on 300 rural married women in Sri Lanka in 1988. We discuss the utility of the design, which is similar to that of an epidemiological surveillance system, for the measurement of coital behavior and pregnancy risk perceptions. Perceptions of pregnancy risk, spousal agreement on sexual relations, menstrual and lactational status, and cycle timing, all measured daily, are found to influence significantly the probability of coitus on that day. The findings suggest that substantial gains in studying fertility regulation are likely from closer investigation of the behavioral connections between motivation for pregnancy avoidance and coital incidence. Comparison of such panel data with those of cross-sectional sample surveys also provides insights into the validity of measures of coital and contraceptive behavior.