Objective: To compare estimates of low birth weight (LBW), preterm birth, small for gestational age (SGA), and infant mortality in two birth cohorts in Brazil.
Methods: The two cohorts were performed during the 1990s, in S o Lu s, located in a less developed area in Northeastern Brazil, and Ribeir o Preto, situated in a more developed region in Southeastern Brazil. Data from one-third of all live births in Ribeir o Preto in 1994 were collected (2,839 single deliveries). In S o Lu s, systematic sampling of deliveries stratified by maternity hospital was performed from 1997 to 1998 (2,439 single deliveries). The chi-squared (for categories and trends) and Student t tests were used in the statistical analyses.
Results: The LBW rate was lower in S o Lu s, thus presenting an epidemiological paradox. The preterm birth rates were similar, although expected to be higher in Ribeir o Preto because of the direct relationship between preterm birth and LBW. Dissociation between LBW and infant mortality was observed, since S o Lu s showed a lower LBW rate and higher infant mortality, while the opposite occurred in Ribeir o Preto.
Conclusions: Higher prevalence of maternal smoking and better access to and quality of perinatal care, thereby leading to earlier medical interventions (cesarean section and induced preterm births) that resulted in more low weight live births than stillbirths in Ribeir o Preto, may explain these paradoxes. The ecological dissociation observed between LBW and infant mortality indicates that the LBW rate should no longer be systematically considered as an indicator of social development.