How accurate are first trimester ultrasounds for a conception date? I need to know which man is my unborn childs father. My due due is 2/27/2011 which was first given to me on July 8,2010 at an ultrasound daing me 6 and a half weeks. Had two more ultrasounds done: One on August 27, dating me 13 and 5 days with my due date still 2/27/2011 and another on October 11, dating me 20 weeks 1 day with my due date still 2/27/2011. If I had intercourse with guy A on May 23 and guy B on June 5, who is the father? Is two weeks difference enough time to be able to tell this? I also have had irregular periods and only have ultrasounds to go by, is the information given to me by an ultrasound accurate enough to determine paternity? THANK YOU!