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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.
Moreover, ultrasound-determined due dates are not accurate. One study used the date established by ultrasound at 16 to 18 weeks to test the validity of dating by the last normal menstrual period (LNMP). It found that as gestational age went past term, positive predictive values for the LNMP declined from 95% to 12%. The authors took this to mean the LNMP was inaccurate, but, of course, the ultrasound date is the problem. Even first trimester measurements have an error bar of +/- 5 days in the second trimester and +/- 22 days in the third.
Few practitioners appreciate the limitations of ultrasound or clinical data. Otto and Platt say the due date should not be changed unless the discrepancy is more than two weeks, yet they see doctors changing a due date by a few days, no trivial alteration if a woman will be induced when she exceeds a certain date.
Some risk does accrue in healthy postdate pregnancies (notably meconium passage and big babies) but it does not follow that we should induce all women. Studies have found that as gestational age goes from 37 to 44 weeks, perinatal mortality and morbidity distribute in a U-shaped pattern. If we try to eliminate postdate pregnancies on grounds of increased complications, should we not equally logically try to delay labor onset in the early-term group?
Henci Goer, Obstetric Myths vs. Research Realities, Bergin & Garvey 1994