When I first got pregnant I was 100% sure of when my last period was, and even my conception date. But when I got my first untrasound they said I was exactly a week behind what I thought I was. Why/how does this happen? Now on my papers it says "small to date fetus" and I'm now 21 weeks and my baby is normal size, maybe even a little bigger than the average weight. Could they have been wrong? Or maybe I ovulated late?