Impact of minimum wage increases on smoking before, during and after pregnancy in the USA: a difference-in-differences analysis of cross-sectional surveillance data.

Introduction: Smoking before and during pregnancy has long-lasting adverse effects for children's health and development. Financial stress is an important risk factor for maternal smoking, but there is little evidence on how economic policies affect tobacco use. We examined whether increases in state minimum wages had an impact on maternal smoking and cessation. Methods: This was a secondary analysis of cross-sectional surveillance data from 46 states between 2004 and 2019. Our analysis sample included women without college education. Outcomes were use of cigarettes in the 3 months before pregnancy, the last 3 months of pregnancy, and in the postpartum period, as well as cessation of smoking during and after pregnancy. For the main analysis, a two-way fixed effects difference-in-differences analysis was used to estimate the effects of state-level minimum wage policies on outcomes, adjusting for individual covariates (age, race and ethnicity, marital status, parity, education and month of birth), state-level economic and tobacco policies, and state and year fixed effects. We also explored robustness to de Chaisemartin & D'Haultfœuille (dCDH)'s heterogeneity-robust alternative estimator. Subgroup analysis examined heterogeneous impacts by race and ethnicity, age, marital status and pregnancy intention. We tested differences by subgroup using interaction models and testing the joint null hypothesis of coefficients. We used the Benjamini-Hochberg procedure to adjust p values for multiple comparisons. Results: The study analysed 421 884 women of whom 28% smoked in the 3 months prior to pregnancy. A 10% increase in the minimum wage was associated with a 0.75 (95% CI -1.32 to -0.19) percentage point decrease in the probability of smoking before pregnancy. Associations were strongest for women who were American Indian/Alaska Native, non-Hispanic White, married and had an unintended pregnancy. Two-way fixed effect results were consistent when using the dCDH estimator. There were no significant associations between minimum wages and smoking during pregnancy or postpartum, or on smoking cessation. The conclusions of the analysis do not change when adjusted for multiple hypothesis testing. Conclusions: Estimates suggest that an increase from the current federal minimum wage of US$7.25-US$14 would be associated with a 17% decrease in smoking prior to pregnancy in a demographic group that is disproportionately affected by poor health outcomes.