How is your due date calculated?
Your due date is calculated by adding 40 weeks to the first day of your last menstrual period (assuming a 28 day cycle).
Your menstrual period and ovulation are counted as the first two weeks of pregnancy. If you deliver on your due date, your baby is really only 38 weeks old.
Your due date is only an estimate
On average, only 5% of births happen exactly on your estimated due date. The majority are born within a week on either side of the estimated due date. A baby is considered preterm if they are born before 37 weeks. Early term if born between 37 weeks and 38 weeks, 6 days. Full term if born between 39 weeks and 40 weeks, 6 days. Late term if born between 41 weeks and 41 weeks, 6 days. Post term if born after 42 weeks.