Due date questions

Hi, I've taken a test which says I'm 4-5 weeks pregnant! However when working it out from my last period, I'm further along than that, possibly nine weeks! Although due to removing the implant, my periods were irregular. I've checked with my GP but have to wait for a scan to be organised, which is normally about 12 weeks. But how do I know my real due date, I want to get planning and don't know how to find out? Any help is appreciated! Xxx