I had an ultrasound on 4/19 dating me at 9wks 4days. when could i have conceived?

Approx 2/25: A sono @ 9-10 weeks has an accuracy of about 5 days. So if a baby measures 9wks 4 days, the actual gestational age of the baby may be between 8wks 5 days and 10 weeks 2 days. If conception dating (based on a last period) matches the sonogram, the dates are confirmed. You would have conceived between 2/20 and 3/1.

Subtract: 7 weeks 4 days. This will give you a rough estimate. Makes some assumptions such as a 28 day menstrual cycle and accurate measurements on ultrasound.