im pregnant and my last mensteral period was 1-24-2009 i had a ultrasound to confirm pregnancy due to the fact ive had mult. misscariages in the past few years they only saw a yolk sac and said i was 5 weeks 3 days and to do another one next week so we did one 10 days later and they say that i was 6 weeks 2 days at that time. how accurate are vag.ultrasound because the 5 week 3 days was right on to when my boy friend and i had sex. i did cheat and slept with some one else 3 days later. what should have been my most likely date of conception?