I had a very small bump at 13 weeks, only really noticable by me. When i got to about 16 weeks + that's when friends started to notice and by 18-19 weeks i suddently popped. Bump is still small for nearly 21 weeks but i've noticed that it stays the same for a while then one day it pops again and i am a little bit bigger.
Only just going out buying maternity trousers now. It's different for everyone though so don't worry if your bump is not a big/small as other girls, one day you could wake up look down and say well, hello there!!