Researchers say parents less likely to follow government advice on lifestyle change if they do not recognize their child is obese.
The team - from the London School of Hygiene & Tropical Medicine and the UCL Institute of Child Health, also in London - reports the findings in the British Journal of General Practice.
The researchers also found that parents are more likely to underestimate their child's weight if the child is male, or if they are black or south Asian or from deprived backgrounds.
They say understanding these variations in the population should help policymakers better target awareness raising and programs to address the problem of childhood obesity.
Over the past 30 years, childhood obesity has been increasing in the UK as in the US, where according to the Centers for Disease Control and Prevention, it has more than doubled in children and quadrupled in adolescents.
Parents more likely to make changes if they can correctly classify children's weight
Evidence that being overweight and obese in childhood is linked to higher risk of premature death and disease in adulthood has led to public health initiatives aimed at getting parents to change children's lifestyles and diet.
However, it has already been suggested that these interventions are unlikely to work unless parents understand the government's official scales for measuring childhood obesity.
Senior author of the new study, Dr. Sanjay Kinra, reader in clinical epidemiology at the London School of Hygiene & Tropical Medicine, says:
"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."
So, Dr. Kinra and colleagues set out to look at the scale of the problem and find out if it was the same in all groups.
The team analyzed data from questionnaires that were filled in by the parents of 2,976 children in five different health regions of the UK that are taking part in the National Child Measurement Programme (NCMP).
The NCMP has found that about one in five children in the Reception year (age 4-5) is obese, and this proportion rises to about one in three by Year 6 (age 10-11).
Nearly a third of parents underestimated their child's BMI category
The new study finds that nearly a third of parents (915 respondents) underestimated where their child's BMI sat on the government's official scale. This scale classifies children as underweight, healthy weight, overweight or very overweight (or obese).
The researchers also found that only four parents regarded their child as being very overweight or obese, despite 369 of the children falling into this category.
Only in cases where children were at the very high end of the obesity scale were parents more likely to put their child in the right category.
Co-author Russell Viner, academic pediatrician and professor at the UCL Institute of Child Health, says:
"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."
The National Institute for Health Research funded the study, which forms part of the PROMISE project that aims to improve the assessment and treatment of childhood obesity through research. Prof. Viner and Dr. Kinra are leading the project.
Physical activity and a healthy diet are two of the areas the government advises parents to focus on to help overweight children reach and maintain a healthier weight.
But schools can also make a difference. For example, Medical News Today recently learned of a trial that found bringing chefs into school kitchens can help make healthy meals more palatable to children so they eat more fruits and vegetables.