All depends on your body frame, body shape, pre-programmed weight! I didn't have a bump till around 24 weeks first pregnancy and with my second it was around 18-19 weeks :)
My first one I carried in the back and she didn't show till 8 months.. This one started showing at about 4 months. And he is making me look like a fatty;) don't stress. You will get you baby bump. Them you'll wish you didn't I knock everything over lol.
I'm 34 weeks and my bump is small. It varies from woman to woman.