Supplemental Material for: The association of neighborhood socioeconomic status with birth weight is not confounded by ambient air pollution in twin pregnancies: a retrospective cohort study

Neighborhood SES was defined using maternal residence at birth and statistical sectors (average area: 1.55 km²) to define annual median neighborhood household income. PM10 and NO2 exposure was estimated using a spatial-temporal interpolation method (Kriging) integrating daily monitoring data and land cover data. Analyses were stratified by placentation: one placental mass or two separate placentas.

Multivariable linear mixed models were conducted for birth weight as outcome, linear regression models for gestational age.</p><p dir="ltr">Results: For twins with two separate placentas, per € 3000, - (~ 1 SD) increase in median neighborhood household income birth weight increased with 46.83 grams. Adjustment for PM10 and NO2 did not change the associations. Part of these associations was explained by gestational age, as the association was reduced to 21.92 grams per € 3000, - increase.

Each € 3000, - increase in annual median neighborhood household income corresponded to a longer gestation of 26 hours, independent of ambient air pollution or other covariates.</p><p dir="ltr">Discussion: Although neighborhood SES and ambient air pollution are correlated with birth outcomes, they act through different paths in this twin cohort. This approach provides insights into the etiology of adverse birth outcomes in a high-risk population, even in high-income settings with universal healthcare access. </p>