The prevalence of obesity in children is increasing. Is this biasing the growth charts that physicians use to assess normal growth? In other words are children that would have been in the 50th percentile before the rise in childhood obesity now in a lower percentile?
You can read more about growth charts here. Here's a quotation about their creation:
The 2006 WHO growth curves for children are based on data from the WHO MGRS, a study conducted during 1997--2003 in six sites: Pelotas, Brazil; Accra, Ghana; Delhi, India; Oslo, Norway; Muscat, Oman; and Davis, California (12). The criteria for selection of the communities included 1) socioeconomic status that does not constrain growth of the child (based on infant mortality rate; prevalence of underweight, stunting, and wasting; subpopulation size; and access to safe water), 2) low altitude (<1,500 m [4,921 ft]), 3) low enough population mobility to allow for a 2-year follow-up, 4) at least 20% of mothers in the community willing to follow international feeding recommendations, 5) existence of a breastfeeding support system (typically in the form of lactation consultants), and 6) existence of a research institution capable of conducting the study (12). The international infant feeding recommendations in effect at the time of the study included exclusive breastfeeding for at least 4 months (although predominantly breastfed infants were also included in the study), introduction of complementary foods by at least 6 months but not before 4 months, and continued breastfeeding for at least 12 months. Study participants were provided breastfeeding support as needed and were counseled on complementary feeding, with an emphasis on timing, energy density, feeding frequency, and micronutrient content.
In addition to the breast feeding requirements, there was a straight cutoff for abnormally large children in relation to height:
Weight-for-length measurements of >3 standard deviations from the overall study median were considered to be outliers and excluded from the final sample....To eliminate the effect of overweight children on the weight distributions in the WHO curves for children aged 24--59 months, weight measurements of >2 standard deviations above the study median were excluded; a total of 226 (2.7%) weight measurements were excluded.
They did a pretty thorough job of making sure that those growth charts have a great deal of scientific backing to them, inasmuch as these charts define 'normal' and 'healthy' for the world's children.
If anything, it makes kids of today show higher on the charts than they would be if a new sample was run for the charts today.
The greater concern, and one that I have heard some parents say loudly and proudly, is that their kids are in the 99th percentile. By comparing the size of their child now to children of approximately 10 years ago, in some cases they are much larger, due to obesity. However, when you look at them in comparison to other children they don't appear to be much larger. One personal observation: Our kids are at about the 75th percentile and they look very skinny by comparison to their peers. However we know them to be healthy/active children. You should also be more concerned with how their height relates to their weight. If they are in the 25th percentile of height...and 99th percentile of weight, it is likely they are heading for a serious problem (barring any extenuating circumstances, like growth issues).
One of the purposes of the WHO Growth chart is to set a standard percentile ranking to identify the actual weight whether the child is considered obese and or underweight. The figures are already set as a standard. For you to be able to get the percentile ranking of the child, it is best a baby use a baby weight percentile calculator, so you would know the rank of the child based on WHO standards. The obese children are already classified based on their weight. The number of obese children is not a dependent variable in the percentile ranking.