The number of births varies markedly by season, but the causes of this variation are not well understood. The proposed explanations include temperature or photoperiod (affecting hormonal concentrations, sperm quality or sexual activity), seasonal variation in pregnancy loss, or cultural factors. In this paper we examined whether birth seasonality is influenced by socio-demographic factors. We used data on all live births registered in the Czech Republic in 1989-1991 (n = 387 496). Differences in the degree of seasonality between socio-demographic groups (defined by maternal age, marital status, education and birth order) were examined by inspection of curves, by comparing coefficients of variations of monthly numbers of births, and by calculating the ratios of the number of births in the 3 peak months (March to May) 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 seasonality was weak in mothers who were < or =19 years or > or =35 years old, unmarried, had low education, and expected their first or fourth or higher order birth. In a multivariate model, all four socio-demographic variables contributed significantly to seasonal variation. These results suggest that the seasonality of births is, at least in this population, strongly influenced by socio-demographic factors.

International Centre for Health and Society Department of Epidemiology and Public Health University College London 1 19 Torrington Place London WC1E 6BT UK

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