Plots head circumference (in centimeters) based on the WHO growth charts for <= 2 years and the CDC growth charts for older than 2. The change in data source creates a slight "jog" in the graph at the 24-month point. Data for children with Down Syndrome are from Palmer, et al. (1992). Head circumference of children with Down syndrome (0-36 months), American Journal of Medical Genetics, 42, 61-67, via Greg Richards at growthcharts.com