The Due Date
The concept of a due date is based on a gestational length established by fiat in the early 1800s. Franz Carl Naegele officially declared that pregnancy lasted 10 lunar months (10 x 28 days), counting from the first day of the last menstrual period). However, when Mittendorf et al. measured the median duration of pregnancy, they found that healthy, white, private-care, primiparous women with well-established due dates averaged 288 days and multiparas averaged 283 days, values significantly different from both Naegele’s rule and each other. Others have found similar results. Mittendorf et al. also cited other studies showing racial differences in gestational length. For example, one showed that black women averaged 8.5 days fewer than white women of similar socioeconomic status.