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 non-smoker. 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 non-smoking mother.