Is 20 inches big for a 15 month olds head? pediatrician isnt concerned but wants a mri if it continues to not follow the growth chart. just wanted a second opinion! he hasnt missed any mile stones or anything?

At 15 months this is at the top (95%) head circumference for age. If a kid was 75-95% for all other measurement I would not be as concerned or if the % remains constant over several assessments, I find measuring one or both parents will also find at least one has a head on the large size. It sounds like the doc is paying attention as I would. I consider it standard of care.

Head circumference (HC) at 15-m-o is considered "big." I would be concerned if HC is out of proportion comparing to weight and and height percentiles, abnormal growth rate (more than 1/2 inch growth per month in the first 4 months, then 1/4 in. per month for the next 8 months).