Why are the low birthweight rates in Brazil higher in richer than in poorer municipalities? Exploring the epidemiological paradox of low birthweight

Summary Socio‐economic disadvantage is usually associated with low birthweight (LBW). However, it has been shown that Mexican Americans, despite being economically less advantaged, present LBW rates that are similar to or lower than those found among white women in the US. This fact has been called ‘the epidemiological paradox of low birthweight’. Natality data from Brazil revealed the existence of a similar paradox: LBW rates are higher in more developed than in less developed regions within the country. In this study, data from two population‐based cohort studies carried out in the nineties, including 2439 births in São Luís, a poor city in north‐eastern Brazil, and 2839 births in Ribeirão Preto, a socio‐economically well‐off city in south‐eastern Brazil, were used to explore this paradox. The method proposed by Wilcox and Russell and a graphic analysis of the frequency distribution of birthweight according to gestational age were used to provide indirect information about possible gestational age misclassification. Contrary to expectations, the LBW rate was higher in Ribeirão Preto than in São Luís (10.7 vs. 7.6%, P