How do due dates work?

Did you know your due date is an educated guess of your baby's birth day? This can be based on the first day of your last period, or the 10-12 week ultrasound. If there is a difference of more than a few days, the due date is usually based on your ultrasound. Even then, some people do experience preterm (less than 37 weeks) labour or birth, deliver at term (37-40 weeks) or may go a bit past their due date of 40 weeks.