Objective: To investigate the incidence of preterm delivery in the Belgian population after implementation of smoke-free legislation in three phases (in public places and most workplaces January 2006, in restaurants January 2007, and in bars serving food January 2010).
Design: Logistic regression analyses on routinely collected birth data from January 2002 to December 2011.
Setting: Flanders, Belgium.
Population: All live born singleton births delivered at 24-44 weeks of gestation (n = 606,877, with n = 448,520 spontaneous deliveries).
Main outcome measures: Preterm birth (gestational age <37 weeks).
Results: We found reductions in the risk of preterm birth after the introduction of each phase of the smoking ban. No decreasing trend was evident in the years or months before the bans. We observed a step change in the risk of spontaneous preterm delivery of -3.13% (95% CI -4.37% to -1.87%; P<0.01) on 1 January 2007 (ban on smoking in restaurants), and an annual slope change of -2.65% (-5.11% to -0.13%; P=0.04) after 1 January 2010 (ban on smoking in bars serving food). The analysis for all births gave similar results: a step change of -3.18% (-5.38% to -0.94%; P<0.01) on 1 January 2007, and an annual slope change of -3.50% (-6.35% to -0.57%; P=0.02) after 1 January 2010. These changes could not be explained by personal factors (infant sex, maternal age, parity, socioeconomic status, national origin, level of urbanisation); time related factors (underlying trends, month of the year, day of the week); or population related factors (public holidays, influenza epidemics, and short term changes in apparent temperature and particulate air pollution).
Conclusion: Our study shows a consistent pattern of reduction in the risk of preterm delivery with successive population interventions to restrict smoking. This finding is not definitive but it supports the notion that smoking bans have public health benefits from early life.