Seasonality of birth weight in Chile: environmental and socioeconomic factors.

PURPOSE: Research suggests a relationship between birth weight and season of birth, but findings vary across countries and underlying factors are not well understood. We examine the seasonality of birth weight and explore alternative hypotheses for its etiology-exposure to environmental factors and varying socioeconomic composition of mothers-in Chile.
METHODS: Birth weight of approximately 5 million Chilean singleton live births 37 of 41 weeks of gestation between 1987 and 2007 were analyzed for seasonality by using regression models with month dummies and parametric sinusoidal specifications. Multivariate models with socioeconomic covariates and interactions across geographic regions examine potential factors accounting for seasonal variation.
RESULTS: Marked 12-month and 6-month periodic cycles were found. The amplitude and phase of the seasonal variation change across geographic regions. In the low-latitude northern region, there is a spring peak and a fall nadir, while in middle-latitude colder regions, a bimodal periodicity emerges with peaks in spring and fall, a pronounced winter nadir, and smaller nadir in the summer. Socioeconomic composition of mothers is found to vary with annual periodicity, but it does not account for the seasonality in birth weight.
CONCLUSIONS: Environmental factors rather than the socioeconomic composition of mothers likely account for seasonal variation in birth weight. The change in periodicity of birth weight across latitudes is consistent with a beneficial exposure to sunlight both early and late in the pregnancy, and a detrimental late exposure to cold temperatures only in areas with low winter temperatures.