Parents with obese children may not be able to recognise that their child is overweight unless they are at very extreme levels of obesity, research has found.
The study found that parents were more likely to underestimate their child's weight if they were black or south Asian, from more deprived backgrounds or if the child was male.
The research, which is published in the British Journal of General Practice, discovered that just under a third (31%) of the parents that took part in the study underestimated where their child's body mass index (BMI) was on obesity scales, which classify children as very overweight (or obese), overweight, healthy weight, or underweight.
Just four parents described their child as being very overweight despite 369 children being officially identified as such and fewer than 1% overestimated their child's weight status.
According to official guidelines, children are classified as overweight at the 85th centile and very overweight (or obese) at the 95th centile.
The team estimated that for a child with a BMI at the 98th centile there was an 80% chance that the parent would classify their child as a 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.
Researchers suggested that if parents cannot identify when their child is overweight, it leads to questions about the effectiveness of current public health interventions which aim to address obesity in the home. Recent research found that a third of children in England are now classed as overweight or obese.
They said potential explanations for parents' underestimations may be fear of being judged, unwillingness to label a child as overweight, and shifting perceptions of normal weight because of increases in body weight at a societal level.
They said evidence suggests that parents who recognise their child's weight status are more likely to perceive potential health risks.
The study was led by the London School of Hygiene & Tropical Medicine and UCL Institute of Child Health, and involved questioning the parents of 2,976 children in five primary care trusts: Redbridge, Islington, West Essex, Bath and North East Somerset, and Sandwell.
The team said the research could help evaluate how effective public health interventions for obesity in children are likely to be in different groups of the population.
Senior author Dr Sanjay Kinra, reader in clinical epidemiology at the London School of Hygiene & Tropical Medicine, said: "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."
Co-author Professor Russell Viner, academic paediatrician at the UCL institute of Child Health, said: "Measures that decrease the gap between parental perceptions of child weight status and obesity scales used by medical professionals may now be needed in order to help parents better understand the health risks associated with overweight and increase uptake of healthier lifestyles."