Baby Mesuring Big

I had my 20 week ultrasound and they said my baby boy is mesuring big like 99.9 percentile Big! They are sending me to a specialist to see if my due date is just wrong or if it really is my baby mesuring very big. Like I was supposed to be 19w and 4days and baby was 16oz and they said I was mesuring 21weeks. Anyone else have this happen? ( I am overweight but dont have gestational diabetes.) A little worried but I was already going to have a c-section due to having one with my first she was only 6lbs and 10oz