In June 2000, the Centers for Disease Control and Prevention (CDC) released new growth chart standards. The previous infant growth charts, which have been in use since 1977, were made by measuring a small number of formula-fed, middle class, Caucasian children from southwestern Ohio.
The new charts are a big improvement, being based on broadly representative national data. You can take a look at the improved growth chart metrics at the CDC’s website here.
And though it’s probably more information than you need, if you’re at all curious about the methods the CDC used to arrive at the new metrics, a detailed account can be found here.