how accurate in dating pregnancies are scans done at 8 weeks?

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how accurate in dating pregnancies are scans done at 8 weeks? could a scan at 8 weeks possibly have been an 11 week fetus and be out by 3-4 weeks. i had 3 scans and they all tied in to 4 days of eachother.

Heya I think they can be out a bit.. I took my pos tests 10weeks ago and was told at scan I'm 8.5weeks along so have to reschedule the dates to a month away as for accuracy it's too early! But same thing with first baby.. Dates meant I also wasn't pregnant when I took tests which obv. I was and am!

No it doesn't.. Fetilisation and implantation take a lil time and sperm surivives about 5days. But I've a friend who hadn't seen her bf for 3weeks around estimated conception! And their child is most definitely his