Early scan dates don't match up with last period :-/

[1moretime]

Hello. I only found out last Monday at the docs that I am pregnant. I had a scan last week as with having polycystic ovaries & underactive thyroid my periods have always been irregular.
Looking on my period logger on my phone my last period was 2nd - 6th Dec. I also may have had a period at the end of December too but haven't logged it.
Anyhow my scan showed baby & sac measuring at 6 weeks 1 day om Wednesday so 6 weeks 4 days today.
How can I only be 6 & half weeks if my last period was December?
I did have on the 22nd Jan a tiny spot of blood when I wiped & that was it.
Anyone help me figure this out?
Going back 4 weeks means I will have conceived at beginning of Feb as they add 2 weeks on don't they??
That would be over a month after my last period. Xx

Perhaps because of ur pcos u didn't ovulate until then

If your period is irregular there's no way to know when you conceived except for the scan (or if you tracked your ovulation at the time). I tracked my ovulation as I also have irregular periods so they counted me two weeks ahead (even though I told them I ovulated on cycle day 28) until the first scan then put it back by 2 weeks again.

Did they see a heartbeat? It might be too early though so don't panic if they didn't. I think you're over thinking things.

Thanks for your replys :)
Yea there was a heart beat & I coukd see outline of a tiny wee embryo.
I have no idea as I was told I wasn't ovulating due to polycystic ovaries.
I am very grateful of this pregnancy as we didn't think we could have any more DC xx