Differential effects of cigarette smoking on birth weight by maternal body mass index.

Links between low birth weight and tobacco exposure in utero are well established, as are associations between maternal body mass index (BMI) and birth weight. This study further develops those relationships. In particular, this article analyses whether high maternal weight acts to dampen the previously established link between tobacco exposure and low birth weight. A retrospective cohort study was undertaken, reviewing the birth weights of 13,473 live singleton pregnancies born at a Sydney regional hospital between 1998 and 2003. Results demonstrated a statistically significant decline in reduced birth weight as BMI increased. That is, as body weight increases, tobacco use has a smaller effect on reducing birth weight. Inversely, the effect on reducing birth weight for each cigarette smoked by leaner women was greater. In effect, the adverse influence of tobacco use on birth weight appears to be modulated by increasing maternal BMI.