Trajectories in county-level Low Birthweight Rates and Associated Contextual Factors in the United States, 2016-2021

Background Infants born with low birthweight (LBW; <2,500 grams) are at increased risk of infant mortality, long-term neurologic impairment, and chronic conditions such as cardiovascular disease and diabetes. Although maternal risk factors for LBW are well documented, national and state level estimates may mask substantial county level heterogeneity in LBW risk and its underlying social and structural determinants. Less is known about how LBW rates vary geographically across metropolitan and non-metropolitan counties in the United States (U.S.) and how county level contextual factors contribute to these patterns over time. Methods This longitudinal study included 18,193,145 singleton births across 3,108 U.S. counties during the study period (2016 to 2021). Birthweight and maternal characteristics were obtained from restricted-use National Center for Health Statistics natality files, and county level contextual variables were derived from County Health Rankings data. Counties were classified as large metropolitan, small metropolitan, or non-metropolitan using the National Center for Health Statistics urban or rural classification scheme. Bayesian hierarchical spatial models were used to estimate smoothed standardized incidence ratios of LBW. Generalized estimating equations with negative binomial regression were applied to examine associations between county level contextual characteristics and LBW rates over time, and interaction terms were used to assess effect modification by metropolitan status. Results LBW rates increased significantly across U.S. counties over the study period, with substantial geographic heterogeneity. After adjustment for maternal and county level covariates, metropolitan status was not significantly associated with LBW rates, and no statistically significant interactions were observed between metropolitan status and time or other maternal or socioeconomic characteristics. Higher county level percentages of Black women, underweight women, women who smoked during pregnancy, women receiving no prenatal care, and uninsured populations were significantly associated with increased LBW rates. Conclusions We observed substantial increase in LBW rates from 2016 to 2021 across the U.S. counties, with marked spatial heterogeneity that was not explained by metropolitan status. County level socioeconomic and maternal health characteristics were key predictors of elevated LBW risk. Findings underscore the need for targeted, place-based interventions addressing structural and socioeconomic vulnerabilities, particularly in communities with high proportions of racially minoritized and uninsured populations, regardless of metropolitan status.