How are socioeconomic inequalities in preterm birth explained by maternal smoking and maternal body mass index: A mediation analysis.

Background: Preterm birth affects between 7% and 8% of births in the UK and is a leading cause of infant mortality and childhood disability. Prevalence of preterm birth has been shown to have significant and consistent socioeconomic inequalities.

Objective: To estimate how much of the association between socioeconomic status (SES) and gestational age at birth is mediated by maternal smoking status and maternal body mass index (BMI).

Methods: Retrospective cohort study of a maternity hospital in the UK. The analysis included all singleton live births between April 2009 and March 2020 to mothers 18 years old and over, between 22 weeks and 43 weeks gestation. We estimate two measures of mediation for four low gestational age categories: (i) The proportion eliminated the percentage of the effect of SES on low gestational age at birth that would be eliminated by removing the mediators, through the Controlled Direct Effects estimated using serial log-binomial regressions; and (ii) The proportion mediated is the percentage of the effect removed by equalising the distribution of the mediators across socioeconomic groups, estimated using Interventional Disparity Measures calculated through Monte Carlo simulations.

Results: Overall, 81,219 births were included, with 63.7% low SES. The risk of extremely (0.3% of all births), very (0.7%) and moderately preterm birth (6.3%) was 1.71 (95% Confidence Interval [CI] 1.29, 2.31), 1.43 (95% CI 1.18, 1.73) and 1.26 (95% CI 1.19, 1.34) times higher in the low SES, compared to higher SES respectively. The proportion of this inequality eliminated by removing both maternal smoking and BMI was 43.4% for moderately preterm births. The proportion mediated for smoking was 33.9%, 43.0% and 48.4% respectively.

Conclusions: Smoking during pregnancy is a key mediator of inequalities in preterm birth, representing an area for local action to reduce social inequalities in preterm birth.