Objectives The majority of mothers do not correctly identify their child's weight status. The reasons for the misperception are not well understood. This study's objective was to describe maternal perceptions of their child's body mass index (BMI) and maternal report of weight concerns raised by a health professional.

Design Prospective, community-based cohort.

Participants Data were collected in 2010 from 450 mothers previously included in a longitudinal birth cohort. Mothers of children aged 6–8 years reported their child's anthropometric measures and were surveyed concerning their opinion about their child's weight. They were also asked if a healthcare provider raised any concerns regarding their child's body weight. Child BMI was categorised according to the WHO Growth Charts adapted for Canada. Descriptive statistics and bivariate analyses were used to evaluate mothers' ability to correctly identify their children's body habitus.

Results 74% of children had a healthy BMI, 10% were underweight, 9% were overweight and 7% were obese. 80%, 89% and 62% of mothers with underweight, overweight and obese children, respectively, believed that their child was at the right weight. The proportion of mothers who recalled a health professional raising concerns about their child being underweight, overweight, and obese was low (12.5%).

Conclusions The majority of mothers with children at unhealthy weights misclassified and normalised their child's weight status, and they did not recall a health professional raising concerns regarding their child's weight. The highest rates of child body weight misclassification occurred in overweight children. This suggests that there are missed opportunities for healthcare professionals to improve knowledge exchange and early interventions to assist parents to recognise and support healthy weights for their children.