Preterm birth: temporal trends and socioeconomic inequalities.

Socioeconomic inequalities in perinatal health have been consistently observed across many highand low-middle income countries. Quantifying and monitoring socioeconomic inequalities in health is an important first step toward reducing health inequity and improving population health. Brazil has long been ranked among the countries with the largest inequalities in both socioeconomic position and health. In this issue of the Jornal de Pediatria, Sadovsky et al. report income inequalities in preterm birth (PTB, <37 completed weeks of gestation) in the city of Pelotas over a 30-year period. The authors estimated the slope index of inequality (SII) and the relative index of inequality (RII) of income in PTB rates among almost all births in Pelotas in 1982, 1993, 2004, and 2011. Relative and absolute health differences between groups provide different and complementary information, which can lead to different conclusions, particularly when the focus is monitoring changes in inequality over a prolonged time period. Proportions of a population within any socioeconomic group -e.g., individuals with a university education -inevitably change over time and may differ by