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Birth weight is the single most significant determinant of infant mortality and the chances of a newborn to experience healthy development. Low birth weight also appears to be related to higher risks of several important chronic conditions, such as ischemic heart disease, non-insulin-dependent diabetes, and cancer in adults. Thus factors that influence in utero growth and birth weight may have a serious effect on health outcomes many years later in life. Analysis of seasonal variations in birth weights may enable us to suggest specific factors that influence this measure. In this review we summarize the literature on seasonal variations in birth weight. Although causes of seasonal variation in developing regions are more clearly understood, it is not yet clear which factors affect apparent seasonal variation in birth weight in developed countries. In our analysis we observed a pattern of seasonal variations in developed countries that differed between low-, middle-, and high-latitude countries, and we suggest several mechanisms that may be responsible for this diversity. Namely, we suggest that in middlelatitude climates, the large annual temperature range may cause low birth weights during summer, whereas in high- and low-latitude regions variations in sunlight exposure between seasons may contribute to low birth weights apparent during winter. Identification of the suggested causal environmental factors may have public health implications in the development of primary prevention programs for low birth weight and macrosomia in developed countries.