INSERM U500, 34093 Montpellier, Cedex 5, France
I read with great interest the paper by England et al. regarding the relation between tobacco exposure during pregnancy and infant birth weight (1). These authors found that a very small part of the variation in birth weight (4 percent) was explained by number of cigarettes smoked per day, even after they controlled for other factors.
Beyond the limitations of the markers of tobacco exposure, this result is not surprising for several reasons. First, only smokers (even at a very low level) were recruited for this study (1), so there was a lack of truly long-term nonsmokers, which may have affected the distribution of maternal cofactors (in particular, nutritional status; see below). Second, the choice of the third trimester for measuring tobacco exposure may be irrelevant, since many women stop smoking during the first 6 months, or at least reduce the amount they smoke, for physiologic or psychological reasons. Therefore, the number of cigarettes smoked during the last months of pregnancy is not representative of the current behavior of the mother and of its effects on her health. In fact, the most important drawback in this survey is the lack of dietary data. It is well known that a mothers nutritional status before and during pregnancy is a very strong predictor of fetal growth (2, 3). In addition, tobacco smoking is generally associated with lower body mass index in women, especially young women (4).
In a prospective survey conducted in France some 20 years ago, we found a strong relation between dietary behavior and smoking throughout pregnancy, with all factors explaining up to 39 percent of the total variance in birth weight (5). The analysis of changes during pregnancy according to smoking behavior (in never smokers, smokers who stopped before the sixth month, and persistent smokers) showed very particular dietary behaviors, inversely correlated to smoking: nonsmokers did not change their eating habits during the first trimester, those who stopped smoking increased their caloric intake by 95 kcal/day, and persistent smokers increased their caloric intake by 200 kcal/day. Caloric intake then decreased in all groups from 3 months to delivery, but significantly less so in persistent smokers than in the others. Consequently, weight gain was higher in smokers than in nonsmokers, which, in the multiple regression analysis, appeared to be a key factor in infant birth weight. In the whole sample, weight gain ranked second after controlling for duration of gestation; among smokers, it was ranked first, two positions before number of cigarettes smoked. As a whole, the difference in birth weight between infants of smokers and those of nonsmokers was only 70 g. Finally, the fetal growth of infants of mothers who smoked was better than expected because of the higher weight gain observed in women in this category.
The results of this study (5) may have been missed by the authors (1) or considered not applicable to the US population. Subsequent studies did not find such a clear interaction between anthropometry and smoking on birth weight (6, 7). Hence, the concern is to assess whether the behaviors we observedwhich seem, up to now, specific to French womencannot be considered as a basis for counseling mothers in order to balance the risk of small babies linked to thinness, especially in persistent smokers.
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