K. Haug et al., Maternal smoking and birthweight: effect modification of period, maternal age and paternal smoking, ACT OBST SC, 79(6), 2000, pp. 485-489
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 ra
ndom sample (n=34,799) of all mothers giving birth in Norway 1970-91. Varia
bles 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-smoki
ng 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 pater
nal 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 fo
r 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% d
uring the study period, there was no significant increase in overall mean b
irthweight.
Implication and relevance of results. The negative effect of maternal smoki
ng 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 whe
n the mother was smoking and might reflect two possible mechanisms: (1) tha
t a smoking mother has a greater cigarette consumption when the partner als
o smokes, and (2) that a smoking mother is less concerned about passive smo
king than a nonsmoking mother.