Maternal smoking and birthweight:: effect modification of period, maternal age and paternal smoking

Objective. To study the effect on birthweight of maternal smoking, and its modification by study period, maternal age and paternal smoking. Design. A retrospective questionnaire based national survey comprising a random sample (n=34,799) of all mothers giving birth in Norway 1970-91. Variables studied were parental smoking during pregnancy, birthweight, maternal age and infant's year of birth. Results. The overall difference in mean birthweight between non-smoking and smoking mothers was 197 g. The difference in birthweight between non-smoking and smoking mothers increased with maternal age from 182 g (<20 years of age) to 232 g (35+ years of age). There was no significant effect of paternal smoking on birthweight when the mother was a nonsmoker. When the mother was a smoker and the father was a non-smoker, the birthweight, adjusted for maternal age, was reduced by 153 g (p<0.005). However, when both parents smoked, the birthweight, adjusted for maternal age, was reduced by 201 g (p<0.0005). Even though the prevalence of paternal smoking decreased by 38% during the study period, there was no significant increase in overall mean birthweight. Implication and relevance of results. The negative effect of maternal smoking on birthweight appears to increase with maternal age. For a non-smoking pregnant woman to live with a smoking partner has little, if any, effect on birthweight. The negative effect of paternal smoking was only observed when the mother was smoking and might reflect two possible mechanisms: (1) that a smoking mother has a greater cigarette consumption when the partner also smokes, and (2) that a smoking mother is less concerned about passive smoking than a nonsmoking mother.