Measuring 3 weeks ahead


I'm 26+5 and went to the ob yesterday, she said she wants a sonogram to check the growth of the baby because at my previous appointment I was measuring big and now I'm still measuring big (30weeks instead of almost 27 weeks) I am negative for gestational diabetes and both of my last two babies were small (6lbs 10oz) is there a chance she's wrong and my belly is just big because this is my 3rd baby? Has anyone here experienced this and had a normal sized baby?