The weeks of pregnancy that the scan produces are dated from the first day of the cycle during which you conceived. So no, you wouldn't have been pregnant 6 weeks ago, if measurements said you were "6 weeks pregnant." Women usually ovulate about 2 weeks into their cycles, so if the fetal measurements indicated "6 weeks pregnant" then you likely conceived about 4 weeks ago.
The 40 weeks of pregnancy are actually 38 weeks of being pregnant, but counted as starting from the first day of your last menstrual period, which is 40 weeks before your due date.
So, it sounds like it's more likely that you got pregnant during sex with your "regular guy." Assuming he is the only one you've had sex with in the last 6 weeks. Consider who you were with about 4 weeks ago to be your most likely candidate(s) as far as paternity is concerned.
The earlier the ultrasound, the more accurate the fetal measurements are in dating the pregnancy.
at 12:31 PM on Nov. 19, 2013