How to determine weeks of pregnancy if irregular period?

Hey mummies! I have PCOS and definitely my period is irregular. I know for regular, you would base the week of pregnancy on your LMP. In my case, how will we determine the weeks of pregnancy? I had my 2nd tvs earlier, and still no heartbeat. If i’m going to based it on when we “did” it, i feel like im only 5-6 weeks now. Help #firstbaby #firsttimemom