Seasonal fluctuations in birth weight and neonatal limb length; does prenatal vitamin D influence neonatal size and shape?

BACKGROUND Birth weight is known to fluctuate with season of birth, however, there is little information about seasonal variation in neonatal anthropometric measures. AIMS The aim of this study was to examine seasonal fluctuations in birth weight and selected anthropometric measures. STUDY DESIGN AND SUBJECTS The birth weight of singletons born after at least 37 weeks gestation was extracted from a perinatal register in south-east Queensland (n=350,171). Mean monthly birth weights for this period were examined. Based on a separate birth cohort, principal component analysis was undertaken on neonatal anthropometric measures (n=1233). Seasonality was assessed by (a) spectral analysis of time series data, (b) monthly and seasonal comparison of outcomes. RESULTS Based on register data, birth weight displayed clear annual periodicity. Birth weight differed significantly when compared by month and season. Infants born in October were the heaviest (3484 g), while May-born infants were the lightest (3459 g; P=0.001). Based on the cohort anthropometric data, three components were identified related to (a) overall size, (b) limb length, and (c) head size and skin-fold thickness. Each of these components displayed significant seasonal variation. In particular, prominent seasonal fluctuations in limb length were identified, with peak limb length associated with winter/spring birth. CONCLUSION Environmental factors that have regular seasonal fluctuation influence both the size and shape of neonates. Animal experiments suggest that prenatal hypovitaminosis D may underlie greater limb length. Because birth weight and limb length are associated with a broad range of important health outcomes, the seasonal exposures underlying these effects warrant further scrutiny from a public health perspective.