Introduction of smoke-free legislation in England was associated with clinically important reductions in stillbirth, low birth weight and neonatal and infant mortality.

Analysing over 10 million births this is, to the best of our knowledge, one of the largest studies to have investigated the impact of smoke-free legislation on early life health and the first to focus on perinatal mortality9. We obtained data from national birth and death registries, which constitute the primary source for the production of English national statistics, thereby minimising risks of individuals being missed. Registration of individual data items attained a high level of completeness, with over 97% of individuals having complete data on all covariates included in the primary analyses.

Our study has a number of potential limitations. We had no individual-level information on maternal smoking status during pregnancy. We were therefore unable to assess whether a possible reduction in maternal smoking following introduction of smoke-free legislation, as observed by others, contributed to the observed improvement in perinatal outcomes10,11,12,13,14. Also, a gestational age indicator was missing from the data, as this information was only recorded as an individual item by the Office for National Statistics from 2006 onwards. It was thus not possible to distinguish between the most common underlying causes of low birth weight: intrauterine growth restriction (i.e. being small for gestational age) and reduced length of gestation (i.e. being born preterm)18. In a recent meta-analysis, we demonstrated that smoke-free legislation was associated with reductions in the incidence of preterm birth and of being very small for gestational age9, suggesting that both mechanisms are likely to have contributed to the observed reduction in low birth weight identified. Similarly, we were unable to assess whether the decreases in neonatal and infant mortality identified may in part have been mediated via a reduction in preterm births9. It is however important to note that although missing data on gestational age and maternal smoking status limited the opportunity for a detailed assessment of possible causal pathways (Supplementary Figure S1), this had no bearing on the validity of our findings in relation to the main hypothesis under investigation.

When interpreting our findings it should be noted that over 50% of employed adults already worked in a smoke-free workplace before the legislation was implemented19. There is thus a risk that our study under-estimated the true potential impact of smoke-free legislation, which may be larger in countries with a lower proportion of smoke-free environments prior to implementation. In this respect, it is also important to note that this study was undertaken in a country with comparatively good early life outcomes when judged against international standards20,21.

National public health interventions typically do not allow evaluation through randomised controlled study designs22. We therefore evaluated the impact of smoke-free legislation by undertaking a quasi-experimental study, accepting consequential limitations in causal inference22. It is in such contexts important to consider other factors that may help inform causal reasoning. Particularly noteworthy is that the reductions in adverse early-life outcomes we identified are in line with previous studies demonstrating particular perinatal health benefits as described above (i.e. reductions in low birth weight, being small for gestational age and preterm birth)11,12,13,23,24,25,26,27,28. Furthermore, the link between smoke-free legislation and these reductions is highly plausible given that both SHS exposure and active smoking are well-established risk factors for adverse pregnancy outcomes and infant death1. Meta-analyses of studies among non-smoking women have shown that SHS exposure during pregnancy was associated with a 1.32 (95% CI 1.07 to 1.63; p = 0.02) times increased risk of low birth weight and a 1.23 (95% CI 1.09 to 1.38; p < 0.001) times increased risk of stillbirth3,4. Furthermore, a dose-dependent inverse relationship between maternal urinary cotinine levels (as a proxy for SHS exposure) and offspring birth weight has recently been described29. Contemporary studies assessing the impact of SHS exposure on neonatal mortality are however lacking4,30. As for active maternal smoking during pregnancy, a reduction in which is highly likely to be on the causal pathway between smoke-free legislation and improved perinatal outcomes (Supplementary Figure S1)1,10,11,12,13,14 there is a strong association with adverse pregnancy outcomes as demonstrated by studies reporting a 36–60% increased risk for stillbirth and a 20% increased risk for neonatal mortality1,5. Observational and experimental studies have found that maternal smoking cessation normalises these risks1,31,32,33. For example, a randomised controlled trial of a counselling intervention that successfully reduced SHS exposure during pregnancy reported a significant improvement in birth outcomes34. We are unaware of other public health interventions or changes in perinatal practice co-occurring with the implementation of smoke-free legislation in England that may have been responsible for such substantial immediate reductions in several key adverse perinatal outcomes.

Although no previous studies have investigated the impact of smoke-free legislation on perinatal mortality, several have studied its effect on birth weight11,12,13,23,24,25,26,35. In a recent meta-analysis of six studies, no significant overall impact on low birth weight could be demonstrated: −1.7% (95% CI −5.1 to 1.6; p = 0.31)9. We identified no new eligible studies in an update of this systematic review and meta-analysis (Supplementary Figure S2), focusing on (very) low birth weight and early-life mortality and following the methods described earlier36 Adding data from the current study to the existing meta-analyses, the overall reductions in the risk of low birth weight (7 studies, >12.1 million subjects: −2.20% [95% CI −4.95 to 0.54], p = 0.115) and very low birth weight (3 studies, >10.2 million subjects: −2.61% [95% CI −12.15 to 6.95], p = 0.591) following introduction of smoke-free legislation were not statistically significant. Of note however, the two studies that demonstrated a significant reduction in low birth weight were performed in countries where smoke-free legislation has been particularly comprehensive and compliance high12. Meta-analyses of smoke-free legislation and adult health have consistently shown that its health impact is larger when legislation is more comprehensive8,37. Additional studies are needed to study whether the same accounts for perinatal outcomes, which could further strengthen the case for WHO recommendations to implement comprehensive smoke-free laws17.

The association between smoke-free legislation and mortality was primarily attributable to a reduction in late neonatal mortality, which likely relates to the recognised association between antenatal smoke exposure and common causes of death in the late neonatal period38 such as necrotising enterocolitis39, bronchopulmonary dysplasia40,41 and sepsis42. We were unable to assess the differential association between smoke-free legislation and specific causes of death (except for SIDS) as this information was lacking from our data. Post-hoc sensitivity analyses indicate that the reduction in early-life mortality following smoke-free legislation was likely related to mechanisms other than improvement (i.e. increase) in birth weight.

Whereas we identified an overall association between smoke-free legislation and reduced infant mortality, we were surprised by the finding that SIDS was not affected1,5. Boldo et al. previously estimated that SHS exposure was responsible for 310–420 SIDS cases annually in Europe, amounting to 1.6 excess cases per 100,000 in 20056. It is possible that our study lacked power to detect small changes in this rare outcome although the point estimate was not suggestive of possible benefit (Table 3). As SIDS is a diagnosis per exclusionem, temporal changes in the diagnostic approach towards unexplained infant death (e.g. changes in the proportion of explained deaths, changes in post-mortem examination rates, inter-observer variation in diagnostic criteria) may have resulted in unexplained variation43,44. Misclassification of SIDS may furthermore have influenced the data45. Of note, a recent multi-country ecological analysis found that higher tobacco taxes, but not smoke-free laws, were associated with significant reductions in SIDS rates46.

The impact of smoke-free legislation on perinatal and early-life health is likely to have been mediated via several routes. A number of previous studies have demonstrated important drops in maternal smoking during pregnancy following implementation of smoke-free legislation1,10,11,12,13,14. In Scotland, for example, maternal smoking during pregnancy dropped from 25.4% to 18.8%12. At the same time, similar reductions in low birth weight were identified among women who smoked and those who did not smoke during pregnancy12, suggesting that at least part of the effect is mediated via mechanisms other than reducing active maternal smoking. Improvements in perinatal outcomes were observed among Norwegian mothers whose workplace became smoke-free13. In Belgium, both the smoking ban in pubs and restaurants, as well as the workplace ban benefitted perinatal health23. Smoke-free legislation has furthermore been associated with important drops in smoking in the home through social norm spreading47,48,49,50. The impact observed in our study is therefore likely to result from a mixture of reduced active smoking and reduced SHS exposure in the workplace, public places and the home environment.

Our findings add an important new dimension to the emerging evidence on the benefits of smoke-free legislation to child health9,51, adding to the already well-established broad range of health benefits among adults7,8. Smoke-free laws are an inexpensive and efficient means to achieve sizeable improvements in population health52. Such laws are supported by the public, with support increasing further (particularly amongst smokers) following implementation53. Considering that only around 15% of the world’s population is currently protected by comprehensive smoke-free laws17 and that low birth weight and perinatal mortality remain the primary causes of childhood morbidity and mortality worldwide18,21,54, accelerated action to implement smoke-free legislation is likely to help save considerable numbers of young lives across the globe and through doing so enhance the much-needed progress in meeting the fourth Millennium Development Goal.

Since the majority of the burden of early-life morbidity and mortality occurs in low- and middle-income countries18,21, there is a particular need for studies assessing the impact of smoke-free legislation in these regions9. Work is also needed to determine the impact of tobacco control policies on marginalised populations in high-income country settings, as variation in maternal smoking across socioeconomic subgroups has been shown to account for approximately one-third of the inequalities in stillbirths and infant deaths55. Finally, research also needs to investigate the differential early-life health impact of varying degrees of comprehensiveness of smoke-free laws (e.g. extending the legislation to outdoor public places, private cars and homes) and different approaches to enforcement and success with compliance8.

In conclusion, we present evidence that implementation of smoke-free legislation in England was associated with substantial perinatal and early-life benefits, with over 5,000 cases of low birth weight and almost 1,500 deaths averted within four years.