Hi I Went For An Ultrasound On Wednesday April 22,

Hi I went for an ultrasound on Wednesday April 22, 2015 and I was told according to my lmp I am 20 weeks, but according to measurements I am 21 weeks. My lmp was on December 1, 2014. Well my issue is I'm not sure who the father is. I was with my current bf towards the end of November 2014 and towards the end of December 2014, but I made a horrible mistake and slept with my ex on December 14, 2014. Now I know there is no way to be certain but who do you think is more likely to be the father? My due date is September 2, 2015.