What is already known about this subject: Maternal smoking during pregnancy likely increase the risk of childhood overweight. Childhood overweight is influenced by socioeconomic characteristics of mothers. Characteristics of child at birth determine the likelihood of overweight.

What this study adds: Children of mothers who smoked 1 year before birth (including pregnancy) were likely to be overweight during school ages than those of mothers who never smoked. Confirmation that socioeconomic characteristics of mothers influence the likelihood of childhood overweight during school age. Smoking cessation should be targeted at mothers 1 year before birth to improve their health status and that of offspring.

Objectives: To examine associations between maternal smoking and overweight among school-aged children and also identify mothers and offspring characteristics that affect children's weight.

Methods: We used data from the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development (SECCY). Childhood overweight was defined as having Body Mass Index (BMI) of 85th percentile or above. Smoking patterns among mothers were assessed by questioning smoking behaviour 1 year before birth of the target child: never or ever smoking. Standardized procedures were used to measure height and weight. Descriptive statistics and generalized estimating equations (GEE) were used for the analysis.

Results: Descriptive results showed that children of mothers who smoked anytime within 1 year before birth were more likely to be overweight and have higher BMI percentile averages. GEE results showed that children of mothers who were ever smokers 1 year before birth were more likely to be overweight (OR = 1.39, 95% CI: 1.01, 1.94) and have higher BMI percentile averages (β = 4.46, P = 0.036) from grades 1 through 6 than those of mothers who were never smokers. Additionally, the level of mother's education and birth weight were significantly associated with childhood overweight.

Conclusions: Confirmed relationships between maternal smoking and overweight among school-aged children have important implications for public health policy because this evidence can be used to enhance smoking cessation 1 year before birth to improve the health status of mothers and offspring.