When to buy maternity clothes?

When to buy maternity clothes?
To any of you who answered my question before i'm happy to say that everything is okay with my little angel and thank you all so much. So now i'm curious as to when i should start shopping for maternity clothes. I know every pregnancy is different, but i seem to be gaining weight fast. I'm 7 weeks and 3 days now, and i'm just curious as to when most people have to start shopping for clothes. Thanks!!
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Maternity Wear|Maternity Clothes