If you enter the information into a due date calculator looking for 21 weeks to
fall at approximately 3/31/09 it gives a conception
date of November 17th, 2009. Ultrasounds can be off at times, but without knowing anything else for sure, the likely conception time would fall in that november time frame according to the calculators.
If you go to : http://pregnancy.about.com/cs/pregnancycalendar/l/blpregcalc.