The rate of preterm births in Belgium dropped significantly following stepwise introduction of anti-smoking legislation, a study in the BMJ has found.

The legislation was introduced in three waves. On 1 January 2006 smoking was banned in public places and in most workplaces, on the same date in 2007 in restaurants and at the start of 2010 in bars serving food.

A cohort of 606,877 singleton, 24-44 week live births was collected, starting four years before the first wave of legislation, and continuing for two years after the third wave was enacted. The primary outcome surveyed was preterm birth. A mass of socioeconomic, demographic and even meteorological data was collected and controlled for in the analysis.

Complex analysis of step and slope changes over time, showed a reduction in risk of preterm birth after each successive phase of legislation was enforced. The second and third phases of legislation were followed by the greatest changes in preterm birth. The second phase was followed by a step change in the risk of spontaneous preterm delivery of -3.13% (95% CI: -4.37% to –1.87%, P < 0.01) and the third phase by an annual slope change of -2.65% (95% CI: -5.11% to – 0.13%, P = 0.04).

No changes in overall, or gestationally correct, birth weight were identified.

The authors concluded that public smoking restriction reduced the preterm birth rate by six in every 1,000 births over the five-year study period.

A general criticism of observational studies is that they are open to confounding. Nevertheless the findings are consistent with studies from Ireland and Finland, and the consistent pattern of reduction in preterm births following successive phases of anti-smoking legislation is persuasive.

It is well established that smoking during pregnancy is linked to adverse pregnancy outcomes for the fetus. This increased risk is seen in both active smokers and in non-smoking women exposed to passive smoke.

My take home message from this study is that attention to passive smoking should be part of the regular and opportunistic pre-pregnancy and antenatal advice I give to patients.

Dr Chris Barclay

• Cox B, Martens E, Nemery B. Impact of a stepwise introduction of smoke-free legislation on the rate of preterm births: analysis of routinely collected birth data. BMJ 2013;346:f441