If i measured at 19w and 4d on my ultrasound of september 8th when did i most likely conceive?

Easy counting: When talking about natural pregnancies, all dates are estimates (plus or minus one week). A pregnancy is 40 weeks the way everyone does the counting. Conception happens when the pregnancy is 2 weeks. Therefore, just grab a calendar and count backwards 17 weeks and 4 days from September 8th, and there you have it! You count back 17 and not 19 weeks, and 4 days, because conception is at 2 weeks.