it doesn't really matter.
so long as the baby is growing and healthy...
(just make sure your gynae is doing his/her job, know what i mean?)
I've got a friend who is skinny like a bamboo and her tummy didn't show till 7months!!!!!
but her daughter is healthy and buzzing! *laugh*
my tummy for my first child didn't show till end of 5th month, which is about 20weeks....there after, the growing rate of my tummy was like an inflatable balloon! *laugh*