The study found that parents are additionally more likely to underestimate their child's weight if they are Black or south Asian, from more deprived backgrounds or if their offspring is male.
The identification of gaps between parental perceptions and official guidelines, and variations seen in different demographics of the population, may help us evaluate how effective public health interventions for obesity in children are going to be in different groups of the population.
Questionnaire responses were collected from parents of 2,976 children in five primary care trusts taking part in the National Child Measurement Programme (NCMP).
The researchers discovered that 31 per cent of parents (915) underestimated where their child's BMI sat on government obesity scales - which classify children as very overweight (or obese), overweight, healthy weight, or underweight.
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Highlighting this discrepancy, they found that only four parents described their child as being very overweight despite 369 children being officially identified as very overweight according to the BMI cut-off.
The team estimated that for a child with a BMI at the 98th centile there was an 80 per cent chance that the parent would classify their child as healthy weight but recognised that parents became more likely to classify their child as overweight when the child had a BMI above the 99.7th centile.
"If parents are unable to accurately classify their own child's weight, they may not be willing or motivated to enact the changes to the child's environment that promote healthy weight maintenance," said senior author Dr Sanjay Kinra, Reader in Clinical Epidemiology at the London School of Hygiene & Tropical Medicine and co-lead investigator of the PROMISE trial.
The study was published in the British Journal of General Practice.