Maternal smoking and its association with birth weight.

OBJECTIVE Maternal smoking has been associated with a reduction in newborn birth weight. We sought to estimate how the pattern of maternal smoking throughout pregnancy influences newborn size. METHODS One hundred sixty pregnant smoking women were enrolled in a prospective study. We collected data on maternal age, education, prepregnancy body mass index, and parity, as well as alcohol and illicit drug use. Cigarette use was defined as self-reported consumption before pregnancy, at the time of study enrollment, and in the third trimester. Statistical analyses were performed based on bivariate correlations and multiple linear regression. RESULTS Of the smoking parameters examined, maternal third-trimester cigarette consumption was the strongest predictor of birth weight percentile (partial r = -0.23, P < .001). For each additional cigarette per day that a participant smoked in the third trimester, there was an estimated 27 g reduction in birth weight. Prepregnancy smoking volume was not significantly associated with birth weight percentile in bivariate (r = -0.06, P = .47) or multivariable analyses. Additional factors contributing to birth weight include gestational age (partial r = 0.69, P < .001), maternal body mass index (partial r = 0.23, P < .001), and parity (partial r = 0.16, P < .004). In total, these 4 variables explain 61% of the variance in newborn birth weight. CONCLUSION Maternal third-trimester cigarette consumption is a strong and independent predictor of birth weight percentile. This supports the hypothesis that reductions in maternal cigarette consumption during pregnancy will result in improved birth weight, regardless of the prepregnancy consumption levels. LEVEL OF EVIDENCE III.