There were 631 794 registered deliveries in Flanders during the study period (2002–11). We excluded 24 917 (3.9%) births which did not satisfy the inclusion criteria (multiple births, stillbirths, or deliveries before 24 weeks or after 44 weeks). Among the remaining 606 877 births, 448 520 (73.9%) were spontaneous deliveries. Detailed characteristics of the study population are presented in the supplementary table on bmj.com. Characteristics for spontaneous and overall deliveries are similar. Among the spontaneous births, 51.4% were boys, median (10–90th centile) maternal age was 29.5 (23.6–35.8) years, and median birth order was 2 (1–3). Table 1 provides details of maternal age, preterm birth, birth weight, small for gestational age, and low birth weight by year. During the study period, a total of 32 123 (7.2%) spontaneous deliveries occurred before 37 weeks of gestation. Of these, 25 010 (77.9%) were mild preterm, 3518 (10.9%) were moderate preterm, and 3595 (11.2%) were extreme preterm. A total of 44 225 (9.9%) spontaneous births were small for gestational age, and 23 570 (5.3%) were low birth weight. Median (10–90th centile) birth weight was 3370 (2835–3950) g among full term infants and 2520 (1490–3150) g among preterm infants.
Table 1 Characteristics of study population by year, Flanders, 2002–11. Values are medians (10–90th centiles) unless stated otherwise
The figure shows the smoothed curve of the crude percentage of spontaneous preterm deliveries by month of birth as an exploratory analysis. From the figure, the preterm birth rate was relatively stable before the first smoke-free legislation, followed by a decline in the year after the introduction of the first phase of the legislation (workplace). The decline persisted after the second smoking ban (restaurants), although this was partially reversed in the first months of 2008. A second sharp decline can be noticed in 2010, coinciding with the third phase of the legislation (bars serving food).
Time trend in rate of spontaneous preterm deliveries (with 95% confidence interval) in Flanders, 2002–11, with vertical lines indicating stepwise implementation of smoke-free legislation
We further studied the impact of the smoking bans by using logistic regression models. Table 2 shows the immediate (step) and gradual (slope) changes in preterm birth risk following the introduction of the different phases of smoke-free legislation, after adjustment for the potential confounders. We did not find effect modification by personal characteristics (infant sex, maternal age, parity), so final models did not include interactions terms with these variables. There was no significant baseline trend in the risk of spontaneous preterm delivery (−0.16% (95% confidence interval −1.18% to 0.86%) per year), whereas the underlying trend for overall preterm deliveries increased (0.99% (0.03% to 1.96%) per year). Although table 2 shows reduced risks after each of the three phases of smoke-free legislation, a comparison of models indicates that the second and third legislation phases (ban in restaurants and in bars serving food respectively) were followed by the largest changes in preterm birth. The model producing the best fit consisted of a step change on 1 January 2007 and a slope change after 1 January 2010 (table 2). The second legislation phase was followed by a step change in the risk of spontaneous preterm delivery of −3.13% (−4.37% to −1.87%; P<0.01) on 1 January 2007, whereas the third phase was followed by an annual slope change of −2.65% (−5.11% to −0.13%; P=0.04) after 1 January 2010. The analysis for overall preterm delivery showed a step change in the risk of −3.18% (−5.38% to −0.94%; P<0.01) on 1 January 2007, and an annual slope change in the risk of −3.50% (−6.35% to −0.57%; P=0.02) after 1 January 2010. To put this in perspective, these changes correspond to a reduction of six preterm births per 1000 deliveries over the five study years (after 2007).
Table 2 Percentage change in risk of preterm delivery in association with the successive implementation of public smoking bans. Values are percentage change (95% CI)
Adding data on education and national origin of the mother (available until 2009) to the final model produced similar estimates for the step change in 2007 for spontaneous preterm delivery (−1.98% (−3.40% to −0.54%; P<0.01)) as well as for overall preterm delivery (−3.23% (−5.62% to −0.77%; P=0.01)). We did not observe significant effects of the smoking bans on the risk of low birth weight or small for gestational age in the population, nor on average birth weight.