The seasonality of live birth is strongly influenced by sociodemographic factors

The number of births varies markedly by season, but the causes of this vari ation are not well understood. The proposed explanations include temperatur e or photoperiod (affecting hormonal concentrations, sperm quality or sexua l activity), seasonal variation in pregnancy loss, or cultural factors, In this paper we examined whether birth seasonality is influenced by socio-dem ographic factors. We used data on all live births registered in the Czech R epublic in 1989-1991 (n = 387 496). Differences in the degree of seasonalit y between socio-demographic groups (defined by maternal age, marital status , education and birth order) were examined by inspection of curves, by comp aring coefficients of variations of monthly numbers of births, and by calcu lating the ratios of the number of births in the 3 peak months (March to Ma y) to the number of births in the 3 lowest months (October to December). We found large differences in the size of the seasonal variation in births by socio-demographic factors. The seasonal variation was highly pronounced in mothers who were 25-34 years old, had higher education, were married, and were pregnant with their second or third child. By contrast, birth seasonal ity was weak in mothers who were less than or equal to 19 years or greater than or equal to 35 years old, unmarried, had low education, and expected t heir first or fourth or higher order birth, In a multivariate model, all fo ur socio-demographic variables contributed significantly to seasonal variat ion, These results suggest that the seasonality of births is, at least in t his population, strongly influenced by sociodemographic factors.