I had sex with a guy on march 5 and march 17, april 26 ultrasound dated by pregnancy 7 1/2 weeks, who is the father? I gave birth december 13

Unsure. If your cycles are normal 28 day cycles with ovualtion occurring at approximately 14 days, the most likely answer is march 17. If periods are not normal and ovulation is unpredictable, it is very difficult to tell.
March 17 Guy is "it" Your encounter on march 17 would have allowed for conception around march 19. Hope this is the right one!