Are girls more susceptible to the effects of prenatal exposure to tobacco smoke on asthma?
Prenatal exposure to tobacco smoke through mother's smoking increases the risk of developing asthma later in life. A recent study suggested that this effect is present only in girls. We explored potential differences in susceptibility between boys and girls. We followed all 58,841 Finnish singleton babies born in 1987 through 5 nationwide registries for 7 years and identified all cases of doctor-diagnosed asthma (ICD-9 code 493). The birth registry provided categorical information on the mother's smoking during pregnancy: no smoking (reference), low exposure (<10 cigarettes per day), and high exposure (>10 cigarettes per day). In girls the cumulative incidence of asthma was 0.0245 in the reference group, 0.0310 in the low maternal smoking group (risk difference = 0.0065; 95% CI = 0.0053-0.0076), and 0.0360 in the high maternal smoking group (0.0115; 0.0096-0.0133). The corresponding cumulative incidences for boys were 0.0405, 0.0501 (0.0096; 0.0089-0.0103), and 0.0522 (0.0117; 0.0091-0.0142). In logistic regression analysis adjusting for confounding, the combined effect of male sex and high maternal smoking (compared with female sex and no smoking) was 112% excess risk. This corresponded closely to what would be expected from the additive independent effects of male sex (67% excess risk) and high maternal smoking (44% excess risk). Effects of maternal smoking during pregnancy on the risk of developing asthma are similar in boys and girls, with no interaction on an additive scale.