Combined effects of maternal smoking status and dietary intake related to weight gain and birth size parameters

OBJECTIVE: To investigate the interaction of smoking status and dietary intake during pregnancy and its relationship to maternal weight gain and birth size parameters. DESIGN: An observational prospective study. SETTING: Free-living conditions. POPULATION: Four hundred and eight healthy pregnant Icelandic women. METHODS: Maternal smoking status, lifestyle factors and dietary habits were evaluated with questionnaires. Intake of foods and supplements was also estimated with a semiquantitative food frequency questionnaire for the previous 3 months. All questionnaires were filled out between 11 and 15 weeks and between 34 and 37 weeks of gestation. Smoking status in relation to optimal and/or excessive weight gain during pregnancy was represented with logistic regression controlling for potential confounding factors. MAIN OUTCOME MEASURES: Maternal weight gain, smoking status, dietary intake and birthweight. RESULTS: Women who smoked throughout pregnancy were unlikely to gain optimal weight or more (OR 0.51, 95% CI 0.27-0.97), whereas smoking cessation in connection with pregnancy ('former smokers') doubled the risk of excessive weight gain (OR 2.03, 95% CI 1.24-3.35). The latter association was no longer significant after adjustment for dietary factors and other confounding factors. Former smokers ate the least amount of fruit and vegetables (fruit: 129 versus 180 and 144 g/day (median), P= 0.038; vegetables: 53 versus 76 and 72 g/day, P= 0.026 for former smokers, nonsmokers and smokers, respectively). Birthweight was lowest among infants born to smokers, but birthweight was similar for former smokers and nonsmokers (3583 +/- 491 g versus 3791 +/- 461 g and 3826 +/- 466 g, respectively; P= 0.003). CONCLUSIONS: Smoking cessation in early pregnancy or pre-pregnancy is not associated with low birthweight. It is, however, associated with excessive maternal weight gain and a low fruit and vegetable intake.