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 pontaneous birth occurs around 40 weeks. In Brazil there is a populational reduction of the GA (left shift), presently around 39 weeks, due to interventions in childbirth. Objective: To analyze the left shift of gestational age (LDGA) in São Paulo City (SP) and in Southeastern Brazil, and associated factors. Methods: Epidemiological descriptive study of LDGA in SP (data from Live birth information sector, SINASC) and in Southeastern Brazil (data from “Birth in Brazil Survey”). Differences in GA were estimated, by type of birth (vaginal or cesarean) and payment source (public or private), using GA distribution 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 (39-406/7) while most cesarean were early term (37-386/7). 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 compared 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.