Bernardo L Horta1, Fernando P Hartwig2, Cesar G Victora1. 1. Post-Graduate Program in Epidemiology- Universidade Federal de Pelotas, Pelotas 96020-220, Brazil. 2. Post-Graduate Program in Epidemiology- Universidade Federal de Pelotas, Pelotas 96020-220, Brazil; Medical Research Council Integrative Epidemiology Unit, University of Bristol, Bristol, UK.