Objectives To examine the relationship between maternal self-reported and measured height and weight, maternal perceived weight status and measured body mass index (BMI), and maternal perceived child weight status and measured child BMI.

Setting and design Population-representative National Longitudinal Study of Children Growing Up in Ireland.

Methods Height and weight of 7655 mothers and their 9-year-old children were objectively measured using standard measurement techniques. Mothers’ perceptions of their own weight status and the weight status of their child were reported. The association between accurate perceptions of the mother for herself and her child was tested.

Results Mothers overestimated their height by a mean of 0.5 cm (SD=2.9), underestimated their weight by a mean of 1.4 kg (SD=3.8), consequently underestimating their BMI by a mean of 0.6 kg/m² (SD=1.7). The majority (60%) of obese mothers correctly categorised their own weight status. Only 17% of mothers of obese children correctly categorised the weight status of their child. Overweight/obese mothers who correctly categorised their own weight status were more likely to correctly categorise their overweight/obese child compared with those who incorrectly categorised their own weight status (44% vs 23%, 95% CI for difference 13% to 28%, p<0.001 for girls; 37% vs 27%, 95% CI 2% to 18%, p=0.02 for boys; significantly greater difference for girls compared with boys, p=0.04).

Conclusions Maternal perceptions of their own weight status tend to be more accurate than maternal perceptions of their overweight or obese child. Overweight/obese mothers who correctly categorised their own weight status were more likely to correctly categorise their overweight/obese child.