Seasonal variation of birth defects in Norway

Background: Seasonal variation in the occurrence of birth defects provides indirect evidence of the causal role of environmental factors, because genetic factors do not exhibit seasonality. Aim: This study was undertaken to assess the seasonal variation of birth defects in Norway. Methods: We conducted a nationwide cross-sectional study of 326,560 births in years 1993 e1998, using information from the Medical Birth Registry in Norway. We applied the Lorenz curve and associated Gini index and its 95th percentiles from 10,000 Monte Carlo simulations to identify specific birth defects and birth defect groups with statistically significant seasonal variation. For identified outcomes we applied logistic regression analysis to quantify deviations of risk in high and low peak months. Results: The Gini index indicated statistically significant seasonal variation (alpha = 0.05) for any birth defect, 0.040 (95th percentile = 0.024), respiratory defects, 0.140 (95th percentile = 0.141), and for Down syndrome, 0.148 (95th percentile = 0.126). Based on logistic regression adjusting for maternal age, parity, centrality, population density, and industrial profile, highest risk for respiratory defect was among infants born in March (adjusted odds ratio [OR] 1.82, 95% confidence interval [CI] 1.33-2.50), and for Down syndrome in February (adjusted OR 1.64, 95% CI 1.21-2.22) compared to risks of infants born in other months. Conclusion: Findings suggest that environmental factors with seasonal variation play a role in the etiology of respiratory defects and Down syndrome. Copyright (C) 2013, China Medical University. Published by Elsevier Taiwan LLC. All rights reserved.