Background Asthma is the most common chronic disease of childhood and is socially patterned. The aim of this study was to explore, for the first time, how maternal smoking during pregnancy and breastfeeding mediate the relationship between socio-economic circumstances (SEC) and longitudinal wheezing phenotypes in the United Kingdom Millennium Cohort Study (MCS).
Methods The study comprises a longitudinal analysis of the MCS, based on 11,141 singleton children who participated in all four sweeps of the study (at ages 9 months, 3, 5 and 7 years). Parental interviews were conducted using a validated asthma questionnaire. Data collected included the occurrence of wheezing symptoms and socio-economic factors, including early life exposure to tobacco smoke and breast-feeding. Two common phenotypes were analysed as the primary outcomes: early wheeze that remits before the age of seven, and persistent/relapsing wheeze over all four sweeps. Relative risk ratios (RRR) for these phenotypes were estimated using multinomial regression, according to maternal education as a measure of SECs. Breastfeeding and maternal smoking measures were then entered into the models, as potential mediators between SECs and wheezing.
Results In this representative UK sample, 17% (n = 1948) of children had early remitting wheeze, and 12% (1393) had persistent/relapsing wheeze. Children of mothers with no educational qualifications were more likely to have both wheeze phenotypes, compared to children of mothers with degree level qualifications or higher (RRR 1.48 95% CI 1.22 -1.80 for early wheeze; 1.31 95% CI 1.04–1.65 for persistent wheeze). Smoking during pregnancy was associated with an increased risk of wheezing, whereas increased duration of breast-feeding was protective. Both potential mediating factors displayed a dose-response relationship with wheeze and were socially distributed. Controlling for maternal smoking during pregnancy and breast-feeding explained the elevated risk of wheezing experienced by children from less advantaged backgrounds (a RRR 1.09 95% CI 0.89 to 1.35 for early wheeze; 1.05 95% CI 0.81–1.37 for persistent wheeze).
Conclusion In this contemporary UK cohort, maternal smoking during pregnancy and breast-feeding fully explained the socio-economic inequalities in common wheezing phenotypes. Policies to reduce the social gradient in these risk factors are likely to reduce inequalities in wheezing and asthma also.