yes if your having twins or more! or if your a little bit further along. The top of the uterus (womb) is not pushed up out of the pelvis untill week twelve therefore no bump should be visable but saying that if you know your own boby you will know whether clothes are begining to become tighter which is possible due to weight gain or water retention. Trust me im a mother of three!!!!!!!!!!!!!!!!!!!!!
If it's your second pregnancy or more it's quite common to get a bump really quickly but I'm on my first pregnancy and was in maternity clothes by 10 weeks because everything had spread out quickly already!
I had a swollen 'pot belly' from 5 weeks, and i was in maternity clothes from about 9 weeks. I think if your slim, it shows quicker, coz i was a size 6-8 and i started showing straight away!