I don't know why they would unless they assume the baby was born full term. If that is what they are assuming, they would not go wrong counting backwards from the birth date.
Generally the reason something like a conception calculator asks for the birth date or estimated due date is because it assumes the woman got that date from an ultrasound in their first trimester. Conception calculators also can be set to compute from the first day of the last period, for women who didn't have an ultrasound.
Yes I understand what you are saying but how come after people have had the baby and want to know there conception date, people ask them when the baby was born and go off that, if you understand what I mean?
No, think of it this way. A baby is conceived, and the mother has a problem in her 33rd week (say she gets into a car accident) and they need to do an emergency C-section. Does that change when the baby was conceived? He's still in the same week he was before the car accident.
Annie, does it matter when your baby comes does that change the the conception date?
Having a period is a pretty strong clue that you are not pregnant at that time. If you want to lay out some dates, you might get more specific answers. You had only unprotected sex? The date(s) with the ex-boyfriend, the date of the first day of your last period, the date(s) with the other guy, the baby's due date or birth date (are you pregnant now or are you talking about an event a while ago?), any ultrasound dates and projected pregnancy length from the ultrasound(s), things like that. But a period coming in the normal way (the difference between a 6-day flow and a 7-day flow doesn't matter) between two candidates for dad usually does rule out the one before the period.