Your stomach may not balloon, but it certainly sticks different than just gaining a few pounds. And I have a real hard time believing anyone would not know they are pregnant. After all, after 6 months, there is a 4 pound baby kicking at your stomach. Elbows grazing over the top, knees to your bladder, a head sitting on your pelvis wrong. It is not something that can go unnoticed, after all, it is pleasantly uncomfortable.
So any girl who tells their rents they didn't know they were pregnant, is either extremely stupid, or lying.