BMC Public Health 2008, 8:171 http://www.biomedcentral.com/1471-2458/8/171 0.78 (95% CI 0.71–0.85)). Providing further evidence for a potential causal relation, another meta-analysis docu- mented a dose-response relationship between duration of breastfeeding and risk of overweight [20]. However, a third meta-analysis investigating breastfeeding (as opposed to formula feeding) and BMI in adolescence or adulthood concluded that publication bias and con- founding (e.g., by maternal BMI) cannot be ruled out [37]. Previous studies showed low birth weight to be inversely related to overweight and obesity [7] and higher birth weight to be positively associated [9,18,33]. However, some studies failed to observe associations between weight at birth and BMI levels in childhood [34,36]. A study from North Germany observed sex-specific associa- tions: in boys, low birth weight was positively associated with obesity and in girls a high weight at birth appeared to be a significant obesity risk factor [35]. With respect to the smoking variables included in the analysis, there was consistent evidence for an increased prevalence of both overweight and obesity when there was smoking in the living place, while smoking during pregnancy was related to overweight, but not to obesity. In contrast to this, both studies performed in Bavarian children and focusing on maternal smoking during preg- nancy found an increased risk for obesity, too [9,12]. While smoking during pregnancy, especially in early preg- nancy, may result in neuroendocrine or other metabolic dysregulation [38], smoking in the living place is most Female Sex Age Other than German nationality Study region (East/West) Study location (urban/rural) Study year Educational level* Living space > 75 m2 Number of persons Single child Attendance of day nursery Smoking in the living place Passive smoking during the first three years of life Smoking during pregnancy Breastfeeding for more than three months Preterm delivery Birth weight** Association of predicted probabilities and observed responses (p-value***) *OR for a unit increase in educational level **OR for a unit increase in birth weight level *** corresponding to Hosmer and Lemeshow Goodness-of-Fit Test Table 4: Predictive model for overweight and obesity (N = 26,777) [all analyses are mutually adjusted for all the listed factors] Overweight OR (95% CI) Obesity OR (95% CI) likely a proxy measure for a set of lifestyles which in turn might be linked to carrying excess fat. As we did not have information on dietary patterns, we were unable to test a possible link between smoking in the living place and, for instance, the consumption of energy-dense foods. Further limitations arise from other methodological issues. The measurement of children's weight and height was based on physical examination and can hence be con- sidered a gold standard. However, although measure- ments procedures were standardised, inter-rater variability cannot be excluded. In the statistical analysis, multicollinearity could lead to biased effect sizes. A posi- tive correlation of variables such as the smoking variables would result in an underestimation of the effect size in the multivariable models. As expected, the smoking variables do indeed show positive correlations (data not shown). The univariable effect sizes would therefore represent overestimations. It is evident that the true effect sizes lie somewhere between the univariable and multivariable estimates. With respect to the study design, the cross-sectional nature of the investigation does not strictly allow to infer causal- ity from the identified associations between exposures and overweight/obesity. However, reverse causality seems unlikely in most of the associations found (for instance overweight/obesity leading to parental smoking or to breastfeeding). 1.45 (1.35–1.55) 1.05 (0.95–1.17) 1.52 (1.39–1.84) 0.96 (0.87–1.06) 0.97 (0.90–1.06) 1.06 (1.05–1.08) 0.87 (0.82–0.92) 0.86 (0.80–0.93) 0.91 (0.86–0.96) 1.16 (1.04–1.29) 0.97 (0.90–1.05) 1.32 (1.21–1.43) 1.02 (0.94–1.11) 1.11 (1.00–1.23) 0.88 (0.81–0.95) 1.53 (1.30–1.80) 1.43 (1.38–1.49) 0.15 1.30 (1.15–1.47) 0.86 (0.72–1.04) 1.60 (1.26–2.03) 1.02 (0.86–1.22) 1.08 (0.92–1.26) 1.10 (1.07–1.12) 0.80 (0.73–0.89) 0.80 (0.69–0.92) 0.91 (0.83–1.00) 1.13 (0.93–1.37) 0.87 (0.76–1.01) 1.46 (1.26–1.70) 1.13 (0.98–1.32) 1.13 (0.95–1.36) 0.82 (0.71–0.95) 1.16 (0.84–1.62) 1.49 (1.39–1.59) 0.53 Page 8 of 10 (page number not for citation purposes)