I don't get pregnancy months, (I've seen them don't need to tell me what weeks are monthes) but why…

I don't get pregnancy months, (I've seen them don't need to tell me what weeks are monthes) but why can't it just be 4 weeks for every month, I don't count using the way they make them. 4 weeks 1 month, 8 is 2, 12 is 3, 16 is 4, 20 is, 5 and so on