Big baby already

Fiana • Pregnancy #3!! Due date March 11th 2017😍 I have 2 boys age 4 and almost 2!
I went to the doctor today my baby boy is supoose to be 18w6d but measuring him he was 8 days ahead and weight is 12oz.(like 20 weeks) He is suppose to be 6.7oz. Should they change my due date? Also had to do a blood test for downs because something that was by his heart.