Lmp 21/08/12 due date 29/05/13 went for 12 week scan to find i was only 9 weeks 3 days and given due date of 18/06/13 when could i of concieved?

Big if...: If the ultrasound was spot-on, ; if you have a reg 28-day cycle, ovulating on day 14, then you would've ovulated ~ 9/25/12 ; likely conceived from intercourse in the 9/20-9/27 time frame. But. That's a lot of ifs. The ultrasound date of 9w3d likely has an error margin of +/- 7-10 days, ; your normal cycle might vary as well. But typically, a 9w3d date means it is 7w3d after conception.