If i count back 40 weeks from my due date is that my conceiving date?

38 weeks: Date of conception is 38 weeks before the estimated due date.

Confusing, isn't it?: Pregnancy is 38 weeks in duration when counted from the time of ovulation. In that 'typical' 28-day cycle, that would be 40 weeks from the first day of the last menstrual period. An early ultrasound can nail down your due date, plus or minus a week or less. Lgromkomd.