Seasonality in birth weight in Israel: New evidence suggests several global patterns and different etiologies

Purpose: To investigate the effect of season on birth weight and whether it is expressed also in fluctuations of proportions of extreme birth weights. Methods: Information about 225,545 singletons born during 1998 to 2004 was obtained from a large health maintenance organization in Israel. We conducted a linear regression analysis of the weight with month and year of birth, sex, maternal age, diabetes, and several meteorological factors as independent variables, which were also incorporated into multivariate logistic regression models to examine the effect of season of birth on the frequency of low birth weight (LBW, < 2500 g) or macrosomia ( > 4000 g). Results: A significant (P <.001) seasonal pattern in birth weights was observed, with a peak in July and a trough in January. Babies born in summer had an OR of 1.12 (95% CI; 1.07 to 1.18) for macrosomia compared with those born in the winter. No such pattern was found for LBW. Conclusions: While in regions of mid-latitude, summer is associated with relatively lower birth weight, possibly because of exposure to cold temperature during early or mid-pregnancy, our data suggest that in Israel, the peak birth weight is in summer, possibly because of increased exposure to sunlight in the last weeks of the pregnancy.