due date and baby weight

my ob has changed my due date multiple time i originally was due february 19 and then the last time time he changed it to was march 10 (there’s been 2 previous changes) at my last ultrasound with him she weighed 2 pounds 3 oz (december) well i scheduled a 3d at a private ultrasound place and they said she weighed 5 pounds 2oz i was only 31 weeks with the new due date when she told me that so i looked it up and her weighing that much should put me at 34 weeks instead of 31 weeks and that would be my original due date did he really change my due date for nothing ?