Season and outdoor ambient temperature: effects on birth weight1

OBJECTIVE: To determine the extent to which meteorologic factors explain seasonality in birth weight in a developed country. METHODS: Recorded birth weights were collected for all singleton live births after 36 weeks of pregnancy in Northern Ireland between 1971 and 1986. Data on daily maximum and minimum temperatures, rainfall, and hours of bright sunshine were obtained from a local climatologic station for the same period. For each birth, mean daily maximum and minimum temperatures, rainfall, and hours of bright sunshine were calculated for the trimesters of the pregnancy. Linear regression models were constructed with birth weight as the dependent variable and month of birth as a predictor variable. Months of birth were entered in the models as dummy variables. Adjustment was made for year of birth, duration of gestation, maternal age, number of previous pregnancies, sex, and social class of infants at birth and for meteorologic variables relating to each trimester. RESULTS: A clear seasonal pattern in birth weight was observed, with lowest mean birth weight in late spring and summer. Adjusted mean birth weights were 25.5 g, 29.6 g, and 31.6 g lower in May, June, and July, respectively, than in January. This seasonal variation occurred in both sexes, and in female births, it disappeared almost entirely after adjustment for mean daily maximum temperature during the second trimester of pregnancy. CONCLUSION: Infants born during late spring and summer are lighter than those born in winter, which might be the result of exposure to low winter temperatures during midgestation. Pregnant women should keep themselves warm during midpregnancy.