Revisiting the 'Low BirthWeight paradox' using a model-based definition.

INTRODUCTION: Immigrant mothers in Spain have a lower risk of delivering Low BirthWeight (LBW) babies in comparison to Spaniards (LBW paradox). This study aimed at revisiting this finding by applying a model-based threshold as an alternative to the conventional definition of LBW. METHODS: Vital information data from Madrid was used (2005-2006). LBW was defined in two ways (less than 2500g and Wilcox's proposal). Logistic and linear regression models were run. RESULTS: According to common definition of LBW (less than 2500g) there is evidence to support the LBW paradox in Spain. Nevertheless, when an alternative model-based definition of LBW is used, the paradox is only clearly present in mothers from the rest of Southern America, suggesting a possible methodological bias effect. CONCLUSION: In the future, any examination of the existence of the LBW paradox should incorporate model-based definitions of LBW in order to avoid methodological bias.