With my first pregnancy I was almost 7mo and barely showed (I could still button my pants/wear normal clothes no problem). The ultrasounds shocked the doctors, since I had, if anything, a 'large for gestational age' little girl growing away.
With my son I was showing / had to wear pregnancy pants in my 2nd month. Of course, with him... by the end I was carrying 40lbs in my uterus (aka as big as a full term quad mom). Since multiples run in my family everyone thought I was having twins or triplets at least. It was annoying.
I was thin to start with both pregnancies but stayed thin with my first and was FAR more muscular (6pack, active duty military). The heavy duty muscles and the fact that my uterus expands upward more than outward anyway reeeeeally hid that pregnancy.