What was the conception date if 20 weeks now?

To determine the conception date, you need to subtract 38 weeks (or 266 days) from the estimated delivery date. Since you are currently 20 weeks pregnant, add 20 weeks (140 days) from the first day of your last menstrual period. Counting back 38 weeks (266 days) from this date will give you the approximate conception date. Remember, these calculations are based on an average pregnancy duration and can vary from person to person.