Big baby or wrong due date?

Sa

I went in yesterday to get my cervix checked and found out I am 1cm dilated and 50% effaced and they could already feel her head with no effort. Even though her head is engaged her butt is still in my rib cage and I am measuring at 39 weeks even though I am only 36. I am going in for a growth scan next week (if I make it that far)!