The first day of your last period is where the OB/GYN will calculate your due date from. Like say you get pregnant near November 5th, your due date would be around August 11th the following year. Also the OB will do an ultra sound to determine how far along you are and to help them figure your due date.
If you can remember the date of your last period they should be able to work out your due date and from that they can work out when you conceived. However, if you can't remember the date of your last period, ultrasounds and measuring the size of your bump can be used to work out how far along you are, so you can work out roughly when you got pregnant