If the technition said 20 weeks and 6 days, it would mean you actually concieved 18 weeks and 6 days before the ultrasound. The technition always adds on the extra 14 days before you concieved to age the pregnancy correctly (ie the days of an"average" cycle which would pass before ovulation)
i did a bit of counting on a calander - which took me a bit of messing about what with your backwards month/day/year american dates !!!!!!!!(we go day/month/year in australia)
then I realised I was looking at a 2005 calander- so eventually I figured all that out, counted back and you concieved on the 27th of january 2001.
Which is what you thought wasn't it?