Confusion about due date

[secretlyhermione]

So I've read a lot online that you calculate your due date based on the last day of your period. The last day of my period was the 17 Jan making this technically my 7th week mark.
Yesterday I had my scan and they dated me at 7weeks3days, which brings us back to the 13 Jan... I'm confused on how they calculate these things given I didn't have sex with my partner until the 25th of Jan onwards. Anyone that can help?!

Its dated from the first day of your last period, not the last day.

You normally ovulate 2 weeks after your period, so you would have had sex around the 27th and conceived, they count from your last period but you're not technically pregnant for those first two weeks.

Also conception would have occurred around 2 weeks after the first day of your period. So the 25th onwards sounds about right.

Sorry, yes it's the first day of your last period, also cycles can vary a bit which can give discrepancies in the dates too

Thanks everyone! Yeah my cycle started the 10th and ended the 17th of Jan - I tracked with Clue so am very sure on the dates, but that all makes sense now.
I guess in that case I'll just use the ultrasound date as the most accurate :)