April 26 an ultrasound dated my pregnancy 7 and half weeks, when did i concieve?

March 19: If you were 7 weeks and 3 days on 4/26 then your last menstrual period should have begun on 3/5 and your date of conception would have been 14 days later or 3/19. If you were 7 weeks and 4 days then our date of conception would have been 3/18.

Around March 19: You probably conceived around march 19th to within plus or minus less than a week. Good luck!