Background The current US context is marked by extreme right–left partisanship, which means that state policies tend to bundle together and are not experienced in isolation. While prior work has leveraged abrupt shifts in single policies to examine the effects of state policy on birth outcomes, we examined a holistic measure that captures political polarisation.

Methods Data were drawn from national birth certificates for 2003–2017 (N=56 770 470). Outcomes included preterm birth, low birth weight, small-for-gestational age and other perinatal health measures. The primary exposure was a composite index of right–left state policy orientation, generated from historical data on 135 state policies. Multivariable regressions were used to estimate the association between state policy orientation and each outcome, adjusting for relevant covariates.

Results Compared with infants born in states with right-leaning policy orientations, those born in left-leaning states had lower odds of adverse birth outcomes (eg, low birth weight: OR 0.95 (0.93, 0.97), preterm birth: OR 0.94 (0.92, 0.95)). Subgroup analyses revealed stronger associations for US-born and White mothers. With the inclusion of state fixed effects, left-leaning policy orientation was no longer associated with lower odds of adverse birth outcomes. Models were otherwise robust to alternative specifications.

Conclusion While left-leaning state policy orientation has protective associations with a range of birth outcomes, the associations may be explained by stable characteristics of states, at least during the study period. Future studies should examine state policy orientation in association with other health outcomes and study periods.