Changing weeks? Help

So I’m 4 weeks pregnant. Last period was Sept 8-12th.

I have no idea when the weeks change, can anyone help me out? Like which day I start week 5, week 6, and so on. I know it will be the same day or the week each week but I can’t seem to put 2 and 2 together to figure out which days it starts on.

Thanks. 💕

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