What Percent of Babies Are Born on Their Due Date?

The vast majority of babies are not born on their anticipated due dates. Studies vary slightly, but most indicate that about 5 percent of babies are born on the date predicted. About 80 percent of all births occur in the range of two weeks before to two weeks after the projected due dates.

More facts about due dates:

• Naegele’s Rule is one approach that is used to calculate a due date. This approach calls for adding one year to the first day of the woman's last menstrual period, then subtracting three months and then adding seven days. For example, if her last menstrual period began on 13 June, the baby's due date would be March 20 of the following year. This rule assumes an average pregnancy length of 266 days, or 280 days from the start of the last menstrual period.


• Another method of determining due dates, called Mittendorf’s Rule, accounts for differences in pregnancy terms between Caucasian and non-Caucasian women as well as between first-time mothers and those who have given birth before. Robert Mittendorf’s research showed that the pregnancies for first-time Caucasian mothers lasted an average of five days more than those for non-Caucasians or women who had given birth before.


• Mittendorf’s Rule uses an average duration of 274 days for the pregnancy of first-time Caucasian mothers. The same process is used as in calculating the due date using Naegele's Rule, except that 15 days are added instead of seven. Thus, the due date using Mittendorf’s Rule is eight days later than using Naegele's Rule. In the previous example, then, the baby's due date would be March 28. For non-Caucasians or women who have given birth before, 10 days are added instead of 15.