Why do women in the private sector have shorter pregnancies in Brazil? Left shift of gestational age, caesarean section and inversion of the expected disparity

Introduction: Gestational age (GA) at birth is the main predictor of newborn health, and spontaneous birth occurs around 40 weeks. In Brazil there is a populational reduction of the GA (left shift), presently around 39 weeks. Objective: To analyze the left shift of gestational age (LDGA) in Sao Paulo City (SP) and in Southeast- ern Brazil, and associated factors. Methods: Epidemiological descriptive study of LDGA in SP (data from Live birth information system, SINASC) and in Southeastern Brazil (data from "Birth in Brazil Survey"). Differences in GA were esti- mated, by type of birth (vaginal or cesarean) and payment source (public or private), using GA distribu- tion curves in weeks. Results: In SP, the peak of the curve for GA for vaginal births was 39 weeks, while for cesarean was 38 weeks. Most vaginal births were full term (39-40 6/7 ) while most cesarean were early term (37-38 6/7 ). In the private sector, there were more caesarean sections and lower GA at birth, with more preterm newborns and 60.4% being born early term, while in the public sector, 58.7% of births were full term, and a much higher proportion reaching 40 weeks. Conclusions: Babies born by cesarean and in the private sector lost one additional gestational week. There was an inversion in the expected disparity, with well-off women achieving poorer outcomes com- pared to the disadvantaged mothers. The use of continuous variables to estimate the IG (in days or weeks of pregnancy lost) can contribute to a better understanding of the Brazilian perinatal paradox.