How Accurate is the Pregnancy Due Date?

We were having a discussion at work regarding the accuracy of due dates and the percentages of how close the due date in relationship to the actual birth date are. Can you give us some insight into this issue?

The due date is rarely the date that the baby is born. It represents 40 weeks from the last menstrual period, or 38 completed weeks of gestation. About 2/3 of all babies will be born spontaneously between one week before the due date and one week after.

Furthermore, it is not unusual for the doctor to have an inaccurate due date. Patients may present late for prenatal care, may not remember their last menstrual period, may have a history of irregular cycles, all of which would yield the wrong due date. Ultrasound performed before 13 weeks can pinpoint the due date to within 5-7 days. All of my patients receive an ultrasound at their first prenatal visit. MANY of them are off by 1-3 weeks from when they think they are due!

Also, modern OB doctors often intervene and deliver babies before the due date. Therefore, I would bet that MOST babies delivered by modern OB doctors are born before the due date (but close to it).

Overdue technically means going beyond 42 weeks. If you ask the mom-to-be, overdue is one day past her due date! Only about 2-4% of babies are born past 42 weeks.