Baby is in the 11th percentile?

How bad is this? Also everyone is freaking me out saying it's my eating habbits,, The doctors say there is nothing I can do for this problem and they beleive he has dandy walker. Rare condition were he has a cyston brain and can make him mentally retarted. No mother to be wants to hear they are the cause for there child being sick... What has been your experience with your children, babys?