Dating scan wrong?

[Kkk87]

Hi ladies, I've not posted for quite some time but I had my "12 week" scan yesterday, I thought I was 12 weeks exactly or 12+2 at the very most. The sonographer however measured me at 13+1 .. when TTC I tracked my bbt and used ovulation sticks and I can't see how it's possible that I am this far on. It would mean I conceived 2 days after finishing my period and my bbt certainly didn't look like I ovulated that day. I know it's not a huge deal if the dates are a week out but how accurate are these scans usually?
Tia

It's just how baby has grown for the first scan I believe. My date moved forward by 5 days. As long as baby is healthy and they have no concerns there's nothing to worry about.

They are accurate to + or - 7 days.

I wouldn't worry. If you go "post dates" you might want to bear in mind that you could actually be slightly behind your EDD from scan, but other than that it's all just normal variation. Dating at 12 weeks via size isn't perfect but it's the best method we have (outside of IVF when exact conception date is known!).