Irregular period

Esmi

So i found out im pregnant but have always had irregular periods so i feel like if i go by my last period to see how far along id be it would be wrong? If i go by my last period id be about three months but that doesnt sound right. So i guess what im asking is should i go by my last period even tho i was irregular? (P.s. im going the doctor Tuesday im just asking because im curious myself)