Association of Smoke-free Laws with Premature Deliveries -- A Multistate Analysis

Background: Tobacco exposure during pregnancy is a risk factor for preterm birth and low birth weight (PTB/LBW). A number of studies from European countries showed that smoke-free laws were associated with reduced PTB/LBW, while most U.S. studies reported an overall null effect of statewide smoke-free legislation on PTB/LBW. Little is known about whether the association with legislation varies across racial/ethnic subgroups.

Study Question: Was the enactment of statewide smoke-free laws associated with reduced risk of maternal delivery hospitalizations for PTB/LBW?

Methods: We applied a quasi-experimental method to compare the rate of delivery hospitalizations associated with PTB/LBW births among mothers from states that implemented smoke-free laws (“treatment”) before and after the legislative implementation, compared with the rate in states not passing the legislation during the study period (“control”). We identified 8,018,715 delivery hospitalizations (identified using ICD-9 and DRG codes) in 10 states from the 2002-13 State Inpatient Databases, with data for at least three years before and after the implementation of smoke-free laws. To ensure a balanced number of treatment and control states, we stratified the analysis into: (1) earlier laws (using 2002-09 data): five states that enacted the legislation before 2009, compared with five states that did not have the legislation during 2002-09, and (2) later law (using 2006-13 data): three states that enacted the legislation in 2010 (which were used as controls in the first stratification), compared with two states that did not have smoke-free laws in place as of 2013.

Considering the lags between legislation enactment and implementation, as well as pregnancy and delivery time, we used three dichotomous variables to evaluate the legislative effects: the first two years, Year 3, and later years post-legislation in treatment states; for the control states, all the implementation indicators were coded as zeros. Logistic regressions were used to estimate the association of smoke-free laws with the rate of PTB/LBW delivery hospitalizations, controlling for maternal characteristics, county-level unemployment rate, federal/state cigarette tax, and state and year fixed effects. The analyses were stratified by maternal race/ethnicity.

Results: 9.2% of delivery hospitalizations were PTB/LBW, with non-Hispanic Black mothers having the highest rate (13.3%) versus non-Hispanic White mothers (8.6%) and Hispanic mothers (8.3%).

Overall, we observed a statistically insignificant association of smoke-free laws with PTB/LBW delivery hospitalization rates. However, among non-Hispanic Black mothers, the implementation of earlier smoke-free laws (enacted during 2002-09) was associated with reduced odds of PTB/LBW delivery in Year 3 (odds ratio [OR] = 0.93; 95% confidence interval [CI]: 0.86, 0.99) and later years (OR = 0.88; 95% CI: 0.81, 0.95), respectively. The association was weaker among Hispanic mothers and was not statistically significant among non-Hispanic White mothers. Findings were similar when evaluating smoke-free laws implemented later (enacted in 2010).

Conclusions: Statewide smoke-free legislation is associated with a lower odds of PTB/LBW deliveries among non-Hispanic Black mothers.

Public Health Implications: State smoke-free laws may be an opportunity to address racial/ethnic disparities in PTB/LBW rates in the United States.