Systematic review and meta-analysis of maternal secondhand smoke exposure and neonatal outcomes

Objectives
To determine the risk of adverse pregnancy and foetal outcomes among nonsmoking women exposed to secondhand smoke (SHS) while pregnant and compare the quality of studies in middle- and high-income countries (MIC and HIC).

Material and methods
To assess the effect of maternal SHS exposure on preterm birth, low birth weight, stillbirth, and orofacial clefts, the authors searched three databases (2004-2015) and applied strict inclusion/exclusion criteria. Crude odds ratios (ORs), adjusted ORs, associated 95% confidence intervals, and raw data were used to estimate pooled ORs using random-effects models. Models were stratified by adjusted/unadjusted data, high study quality vs. low quality, and country income level.

Results
The authors identified 39 studies that assessed the effect of maternal SHS exposure on the four outcomes. Of these studies, the 20 conducted in MICs were of lower quality on average than those conducted in HICs. Evidence of a statistically significant effect of SHS exposure was found using adjusted pooled ORs among the high-quality studies for low birth weight (pOR: 1.282 [95% CI: 1.103-1.490]), preterm birth (pOR: 1.290 [95% CI: 1.127-1.476]), and stillbirth (pOR: 1.230 [95% CI: 1.037-1.459]). Using the high-quality adjusted ORs, the effect of SHS exposure in nonsmoking pregnant women on cleft palate with or without cleft lip did not reach significance (pOR: 1.307 [95% CI: 0.893-1.9]).

Conclusions
In all analyses, the odds of having negative outcomes were greater among studies conducted in MICs than in HICs. More research is needed in middle-income countries to inform policy and practice.