All About Due Dates

Why Are Due Dates "Estimates?"

Any due date is simply an estimate.  Even though the average length of a pregnancy is 280 days, few pregnancies are exactly 280 days in length.  For any average, there is a range on either side of that average that is still considered normal.  For example, the average height of men in the United States is 5 feet, 10 inches.  But, most perfectly normal American men are taller or shorter than 5 feet, 10 inches.  By the same token, even though the average length of a pregnancy is 280 days, most normal pregnancies are longer or shorter than 280 days.  However, about 80% of all pregnant women with a reliable last menstrual period will deliver within 14 days either side of 280 days from the first day of their last menstrual period.