The estimated due date that obstetricians provide assume that pregnancy lasts 40 weeks. But there is no timer that goes off at 40 weeks that signals that labor should occur — it is considered normal to give birth up to two weeks before or after the due date. Determining the day that the pregnancy began is also imprecise and makes predicting a due date that much more challenging.
A study of 18,708 births in Australia found that a mere 5% of babies were born on their estimated due date, and only 66% occurred within seven days. Here’s a histogram from the study showing the number of days between the date of birth and the due date.
Similarly, here’s a chart showing the length of pregnancies in the U.S. over a three year period: