If your last AF was on the 17th September then you are 10 weeks approximately (on a 39 day cycle), depending on ovulation. If you definitely conceived on the day you think you did then I think it would be +2 days.
Doctor's calculate it from your last menstrual period (lmp) because it's easier to pinpoint.
LMP dates are just used as an estimate by doctors as most people know when their last period was but not everyone knows when they ovulated. They are based on a 28 day cycle in which you ovulate roughly 2 weeks in though, so if you have longer cycles and know when you ovulated/conceived you would add 2 weeks to the number of weeks since conception x
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