7 wks on 7-28-17 due date march 31st 42 weeks did they add on 2 weeks march 17th is 40 weeks when did i possibly conceive??

38 weeks before due: Pregnancy is counted as 40 weeks, and conception is 38 weeks before the due date. One can use a calendar to count back 38 weeks from the due date set by the ultrasound scan done at 8-11 weeks of pregnancy, to see the estimated conception date. Scans done at 8-11 weeks are quite accurate at ESTIMATING pregnancy dates. The exact date of natural conception is never known, and rarely matters.