When does your 6th month start?? Kindly help

Viya
Hi i am 21 weeks and i know its my 5th month going on. I want to know when does your 6th month start? As per internet it says from 22nd week but as per the famous book ' What To Expect When You're Expecting' it says from 23rd week? 
I am confused ?? Kindly help 

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