MATERNAL COTININE LEVEL DURING PREGNANCY AND BIRTH-WEIGHT FOR GESTATIONAL-AGE

Background Recent studies have found that cotinine is a better predict or of birthweight than the number of cigarettes smoked in pregnancy. I n this paper we test this hypothesis and use cotinine to explore the e ffect of environmental tobacco smoke (ETS) on birthweight. Methods In all, 1254 white women were interviewed at booking, 28 and 36 weeks abo ut the number and brand of cigarette smoked. Cotinine was assayed from blood samples taken on the day of interview. The outcome was birthwei ght for gestational age. Results There was good agreement between self -reported smoker/non-smoker status and maternal cotinine with 1.3% wom en mis-reported as non-smokers at booking, 0.6% and 1.8% mis-reported at 28 and 36 weeks respectively. Among smokers, cotinine was more clos ely related to birthweight than the number of cigarettes smoked at all three time points (r = -0.25 versus r = -0.16 at booking). A reductio n in cotinine between booking and 28 weeks was associated with increas ed birthweight but the effect was not statistically significant. Among non-smokers the association between birthweight and cotinine was not statistically significant after adjusting for maternal height, parity, sex and gestational age. Difference in mean birthweight between non-s mokers in the lower and upper quintiles of cotinine was 0.2% (95% CI:- 2.4, 2.8). Pooling the results of 10 studies plus our own gave an esti mated difference in mean birthweight between women unexposed and expos ed to passive smoke of 31 g (95% CI: 19, 44). Conclusions Cotinine is a better predictor of birthweight than the reported number of cigarett es smoked. If biochemical analysis is impossible, then self-reported s moking habit should be obtained prospectively using a structured appro ach. Any effect on birthweight of maternal passive smoking during preg nancy is small compared with the effects of maternal active smoking.