Modeling Seasonality in Fecundability and Births

This paper uses a model of seasonal fluctuation in fecundability and births to analyze the plausibility of several proposed explanations of birth seasonality. We begin with a model of fecundability at the individual level that combines behavioral and biological components, with particular attention to the role of coital frequency, sperm concentration, and fetal loss. The individual level model is expanded into a model of seasonal fluctuations in total births at the population level, explicitly accounting for seasonal fluctuations in the size of the susceptible population. Simulation of the model suggest that fluctuations in the components of fecundability in response to extreme heat could plausibly generate changes in conception rates similar in magnitude to observed patterns. Little independent evidence exists, however, to confirm the effects of extreme heat on the components of fecundability.