In my previous post, I talked about how I compared my childhood growth to the Center for Disease Control’s 2000 growth charts. Some questions I had while doing this:
- what was the CDC’s methodology?
- can I compare myself with the 2000 charts or should I do it with an earlier version?
- who do these percentiles represent? (e.g. if it’s only Caucasian, middle-class kids then it’s not very representative)
Turns out the National Center for Health Statistics (NCHS) had published charts in 1977. You can still reconstruct the percentile curves from here. But this article explains how the 1977 NCHS charts are outdated because of their sampling and statistical methods.
The NCHS growth charts became outdated for a number of reasons. Although the charts for children 2-18 years of age were based on a nationally representative sample, children in the 1960s and 1970s were less likely to be breast fed, and formula composition was different. Even more problematic, however, were the NCHS charts for infants 0-24 months of age. Those infant charts were standardized from the Fels Longitudinal Study of Ohio children using data collected between 1929 and 1975 (Dewey, 1998). They were limited because they were normed from a small sample of formula-fed, Caucasian, middle-class children. In addition, interval growth measures in the Fels study were made too infrequently to adequately describe the trajectory of growth (Ryan, Roche, & Kuczmarski, 1999).
The new 2000 CDC growth charts are significantly different (available at www.cdc.gov/growthcharts). First, they are developed from data that are more representative of the nation’s ethnic diversity and current feeding practices (Ryan et al., 1999). Second, statistical methods incorporated frequent interval measurements that were used to smooth the curves more accurately than was possible in the 1977 charts. The 2000 CDC charts also exclude very low birthweight premature infants (born at less than 1500 grams) so as not to skew the data with outliers. Finally, the new charts include percentile curves up to age 20 years and added the 3rd and 97th percentiles to aid in tracking growth of children at the extremes of the normal distribution.
So what are the differences between the 1977 NCHS and the 2000 CDC charts?
Because the process for making the new growth charts was considerably different, the percentile values have changed slightly. U.S. children, particularly boys, are somewhat taller and heavier than children were in the 1950s and 1960s, thus slightly increasing the percentiles for height and weight at some age intervals on the 2000 charts. Children may, therefore, plot at slightly lower percentiles on the new curves compared to the 1977 versions. Nurses must take note of this if switching a particular child’s measures from the 1977 charts to the 2000 charts because their growth might plot into slightly lower percentiles on the new charts. Finley, Lindeke, and Rogers (2001) found a small, but statistically significant difference in boys’ height percentile measurements between the 1977 and 2000 growth charts.