scan at 6 weeks, accurate dating?

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Hi ladies, those of you who had a scan between around 6-7 weeks and that's what the measurements showed up... were your dates the same at your 12 week scan?
E.g. I have had a scan when I thought it was 8 weeks and they said 6+3.. she said it wouldn't be totally accurate.. so I wonder what it will be at 12 weeks if it'll work out further along or behind! Just wondering about how accurate everyone else's was