I'm 32 weeke with my second girl (as far as we've been told anyway)
i 've carried low with both of them, and this time 'round I have that spare tyre effect too !!!!!!!
You really can't tell from the belly- how you carry depends on your body type, height, baby's position, weight gain and so many other factors that you can never tell just by looking.
I'd trust the scan. Although..... I know a woman who had two scans, one in england, and one in auistralia, and was told on both occasions that she was having a boy. She had a girl.
So buy some clothes in white, or yellow, just in case !!!!!!!!!