Ultrasound conception date, who is the father?

My ex-girlfriend is pregnant and went in for her first ultrasound on 12/20. The baby measured 3.87cm from crown to rump, which calculated to 10 weeks 5 days pregnant. It gave her a 'conception' date of October 5th, and a due date of July 11th 2008. Her last period started Oct 1st and she has a 31 day cycle.

The last time we had sex before we broke up was October 7th, and she slept with another guy for the first time on October 19th. Also she was on birth control at least until we broke up, and maybe after.

I realize it is possible that I am the father, but how likely is it? It seems like 19th fits a lot better with when she should have been ovulating and her due date. I plan on asking for a paternity test to be sure, but how much should I worry?

Related Information

• Could I Finally Have Figured It Out? Do You Think All These Signs Add Up To One Thing? - sex position - Could - Figured - Finally - Have - Signs - These - Thing - Think - 69 866
• Pregnancy And Ovulation – How Ovulating Leads to Pregnancy
• I read a book about gender selection that claims the following: (is it true?)? | How To Have A Baby Boy