Prevalence and risk of stillbirth according to biologic vulnerability phenotypes in the municipality of São Paulo, Brazil: A population-based cohort study.

Objective: To estimate the prevalence and risk of stillbirths by biologic vulnerability phenotypes in a cohort of pregnant women in the municipality of São Paulo, Brazil, 2017-2019.

Methods: Retrospective population-based cohort study. Fetuses were assessed as small for gestational age (SGA), large for gestational age (LGA), adequate for gestational age (AGA), preterm (PT) as less than 37 weeks of gestation, non-PT (NPT) as 37 weeks of gestation or more, low birth weight (LBW) as less than 2500 g, and non-LBW (NLBW) as 2500 g or more. Relative risks (RR) with robust variance were estimated using Poisson regression.

Results: In all 442 782 pregnancies, including 2321 (0.5%) stillbirths, were included. About 85% (n = 1983) of stillbirths had at least one characteristic of vulnerability, compared with 21% (n = 92524) of live births. Fetuses with all three markers of vulnerability had the highest adjusted RR of stillbirth-SGA + LBW + PT (RR 155.00; 95% confidence interval [CI] 136.29-176.30) and LGA + LBW + PT (RR 262.04; 95% CI 206.10-333.16) when compared with AGA + NLBW + NPT.

Conclusion: Our findings show that the simultaneous presence of prematurity, low birth weight, and abnormal intrauterine growth presented a higher risk of stillbirths. To accelerate progress towards reducing preventable stillbirths, one must identify the circumstances of greatest biologic vulnerability.