Objective To estimate whether the incidence of low birthweight and rates of infant mortality were associated with Massachusetts health reform in the overall population and for subgroups that are at higher risk for poor health outcomes. Data Sources Individual‐level data on birthweight were obtained from the National Center for Health Statistics detailed natality files, and aggregated county‐level mortality rates were generated from linked birth–death files. We used restricted versions of each file that had intact state and substate geographic identifiers. Research Design We employed a quasi‐experimental difference‐in‐differences design. Principal Results We found small and statistically nonsignificant associations between the reform and the incidence of low birthweight and infant mortality rates. Results were consistent across a number of subgroups and were robust to alternative comparison groups and alternative modeling assumptions. Conclusions We found no evidence that the Massachusetts reform was associated with improvements in individual low birthweights or county‐level infant mortality rates, despite increasing health insurance coverage rates for adult women of child‐bearing age. Because our mortality analysis was ecological, we are not able to draw conclusions about how an individual‐level health insurance intervention for uninsured pregnant women would affect the mortality outcomes of their infants.