conception date help

So I'm 36 weeks preggo. My lmp was jan 25 2011 so with that date I used calculator and conecption date was 2/8/11. Me and my husband had sex that day the day b4 and the day after that unprotected and I told him I'm ovulating so let's be careful but of course we weren't he didn't pull out. Before those days we hadn't had sex since 1/31/11 which according to my calculator I wasn't ovulating or anything on that day but it was like a quick 1 min thing that I made him stop so he pulled out. My ? Is which date did I conceive really 1/31 or 2/8. I went to dr and they did ultrasound and said I was right on with my dates and baby was due exactly on 11/1/11 and they determined I was exactly 21 weeks just like I calculated. Well anyway any help would be greatly appreciated!!

[hudsy]

if you're normally regular you go 40 weeks from the first day of your lmp so that puts you right at 11/1/11. Pulling out isn't preventing anything and sperm can live 4-5 days