The fundal height is not exact, so as haroula said dont stress about it. The measurement can be affected by bubs position, amount of fluid and other things. I think I was always measuring "behind". I too was told by people "omg youre 30 weeks?? no way! youre tiny" and stuff like that, but then other people would ask if I was carrying twins because they thought I was huge... Anyway, I have a perfectly healthy baby who was a good weight and size at birth.