Body Mass Index Categories in Children

The questions used to define the indicator were: How tall is child without shoes? How much does child weigh without clothes or shoes? If unsure: As you were unsure or did not know the weight of child would you be able to measure child and provide us with that information when we ring you back in about a week's time?

Body Mass Index (BMI) is calculated as follows: BMI = weight(kg)/height(m) ². 

For 18 years and over, the BMI scores are underweight (BMI under 18.5), healthy weight (BMI from 18.5 to 24.9), overweight (BMI from 25 to 29.9), obesity class I (BMI from 30 to 34.9), obesity class II (BMI from 35.0 to 39.9), and obesity class III (BMI of 40 and over).

For children and adolescents, the same categories are used but are linked to international cut off points defined by sex to pass through a BMI of 16, 17, and 18.5 for underweight, 25 for overweight, and 30 for obesity at age 18 years (Cole et al. 2000; Cole et al. 2007).

The validity of self-reported height and weight has been investigated in adult, adolescent, and young adult populations. While many studies have observed a high correlation (96 per cent agreement) between BMI calculated from self-reported and measured height and weight, there is ample evidence that self-reported height and weight is not as exact as measured height and weight.

A  study of NSW Population Health Survey data found that parent-reported height measurements were consistently lower compared with SPANS measurements. Height growth cut points obtained from the US Centers for Diseases and Prevention (CDC) were used to quantify the proportion of the samples with extreme observations by sex and age. For height, 10.5% of the sample had a parent-reported height less than the 1st percentile cut point, whereas in the SPANS sample this was 0.5%. Also, at the other extreme, parent-reported height exceeding the CDC 99th percentile was 5.6%, compared with 2.0% in SPANS.  This leads to an overestimation of  overweight/obesity prevalence in children.  To derive estimates for the NSW population, children whose parent-reported height falls below the 1st percentile, or above the 99th percentile of the  height growth cut points from the CDC, were excluded.

References

Cole T, Bellizzi M, Flegal K, Dietz W. Establishing a standard definition for child overweight and obesity worldwide: International survey. BMJ 2000; 320. Available at https://www.bmj.com/content/320/7244/1240 (Accessed 13 May 2021).

Cole Y, Flegal K, Nicholls D, Jackson A. Body mass index cut offs to define thinness in children and adolescents: International survey. BMJ 2007; 335(7612): 194. Available at https://www.bmj.com/content/335/7612/194 (Accessed 13 May 2021).