K the first day of my last period was September 30, 2010. It is now February 3, 2011 and I am 15 weeks pregnant as per my ultrasound. Is there a way to find out when I conceived?

Count back 15 weeks. Then add (approximately) 14 days to that and that's around the time you would have ovulated.

It's not perfect science - you may never know the exact date.