not sure who the father of my baby is

The day of my last menstrual period was december 10th and it ended on december 14th. On december 15th and december 17th I had unprotected sex with a friend but he did not ejaculate in me. I did not have sex again until december 24th, this time it was with my husband.  We had unprotected sex 2 times that night and he ejaculated in me both times. I researched things online and they all said my most fertile day was the 24th. When I went for my first ultrasound, the doctor had prewarned me that the date of conception may be off cause I was farther along in my pregnancy than other women. My ultrasound was on march 16th and the computer said I was 13 weeks and 1 day. I found that to be highly inaccurate cause 13 weeks and 1 day ago I was still on my period. I need some guidance as to whom is my babys daddy, my friend? Or my husband? Could the ultrasound machine been off and could I just have a larger than normal child? Please help.