I am concerned that the milestone data in this paper1 are not based on a representative population sample. The data published for 75% of the population reaching the milestones listed in Tables 3 and 5 seem to reflect a data set based on the most delayed 20% to 25% of the population with Down syndrome. It seems that this clinic population is skewed to those with more medical issues, more severe developmental delays, and behavioral difficulties. I believe these milestones to be misleading, risk lowering practitioner expectations, and could cause harm to this population and their families.