The association between the intensity and duration of cigarette smoking during pregnancy and the frequency of low birthweight, preterm births and intrauterine growth retardation was investigated in a historical cohort. All 5166 livebirths occurring in the city of Pelotas, Brazil, during 1993 were identified and mothers interviewed soon after delivery. Children whose mothers smoked during pregnancy had a birthweight 142 g lower than those of non-smoking mothers. The odds ratio for low birthweight among children of smokers was 1.59 [95% CI 1.30-1.95]. There was no association between smoking and preterm delivery assessed by the Dubowitz score. In relation to intrauterine growth retardation, smoking was associated with an odds ratio of 2.07 [95% CI 1.69-2.53]. There was a direct dose-response association between the number of cigarettes smoked and the risk of growth retardation. Women whose partner smoked were also at higher risk of having a child with growth retardation. All the above results were adjusted for confounding factors. The effect of maternal smoking on low birthweight seems to be attributable to intrauterine growth retardation rather than preterm delivery.
PIP: Although the deleterious effect of maternal smoking on birth weight has been well documented, no study has investigated the impact of smoking on the different combinations of low birth weight (LBW), preterm delivery, and intrauterine growth retardation (IUGR). The present study addressed these associations through a cohort analysis of virtually all 5166 live births occurring in Pelotas, Brazil, in 1993. Mean birth weight was 3169 g; the prevalences of LBW, preterm birth, and IUGR were 9.1%, 8.0%, and 8.9%, respectively. The prevalence of smoking at conception was 33.2%; 26.2% of mothers smoked during the entire pregnancy; and 43% of mothers' partners smoked. The infants of mothers who smoked during pregnancy weighed an average of 142 g less at birth than those of nonsmokers. Logistic regression analyses of LBW were adjusted for social class, maternal education, parity, pregnancy interval, prior LBW, maternal height, and number of antenatal care visits. Mothers who smoked for part or all of the pregnancy were 1.59 times more likely to deliver a LBW infant than nonsmokers. There was no association between maternal smoking and preterm delivery. The risk of IUGR was 2.07 times higher in mothers who smoked; women who stopped smoking during the first trimester, however, had a risk similar to that of nonsmokers. Smoking by the mother's partner also increased the risk of IUGR (odds ratio, 1.33). Smoking was associated with at least a doubling of risk of IUGR, whether or not LBW or preterm birth was also present, suggesting that IUGR is the key factor mediating the effect of smoking on birth weight.