need a little of an opinion

ok so I'm currently 20 weeks an 1 day. I'm due the 10th of Dec. I was with my boyfriend of 5 years toward the beginning of March up until about the 10th. we broke up and on March 19th, I had sex with someone else. I had my last period on Feb. 24th, start date. I found out in April I was pregnant. my first ultrasound was may 1st and I was 9 weeks 3 days. I'm just wondering if there's any way you tell who is most likely the father. the Drs have gave me a conception date of March 10th, and they say that its accurate with how far along I am. I just need opinions on this?