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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.

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

Go figure! Try this method to calculate your due date

Wood’s method: Carol Wood, Yale nurse-midwifery professor, came up with a method to calculate the due date that takes into account individual variations in the menstrual cycle as well as the effect of a woman’s having had previous pregnancies.

1. Add 1 year to the first day of the last menstrual period, then

  • For first-time mothers, subtract 2 months and 2 weeks
  • For multiparas, subtract 2 months and 2.5 weeks (18 days)

2. Add or subtract the number of days her cycle varies from 28 days

  • 1st-time mothers with 28-day cycles: LMP + 12 months – 2 months, 14 days = EDD
  • Multiparas with 28-day cycles: LMP + 12 months – 2 months, 18 days = EDD
  • For cycles longer than 28 days: EDD + (actual length of cycle – 28 days) = EDD
  • For cycles shorter than 28 days: EDD – (28 days – actual length of cycle) = EDD

EDD: Estimated day of delivery

LMP: Last menstrual period

Anne Frye, Holistic Midwifery Vol. 1, Labrys Press 1995

The Due Date Dilemma

A recent report in the OB/Gyn Journal December 2001 states that eliminating the concept of a due date, “may be helpful to all involved.”

The process of calculating due dates may be flawed as not all women ovulate 14 days from the onset of their menstruation. Additionally, other health factors of the mother play a role in delivery time. In reality, only 5% of all babies are born “on schedule”, anyway.

Because of the due date women feel pressured, become anxious and are led into inductions by their practitioners. Inductions usually lead to further interventions in birth. Interventions in birth frequently lead to trauma for both the mother and baby.

Dr. Vern Katz suggests that doctors expand the concept of a due date to a “due week.” In doing so, “it may allow biology to take its course a bit more.”

Katz VL, Farmer R, Tufariello J, Carpenter M Why we should eliminate the due date: a truth in jest Obstet Gynecol 2001 (Dec); 98 (6): 1127-1129

A prospective study was conducted at a West German US Army Hospital to compare the accuracy of fetal weight estimation by a physician’s clinical estimate as compared to ultrasound. One hundred women had Leopold’s and vaginal examinations, an estimate was made. Then the same examiner performed an ultrasonic estimation of weight. The exam was done within 48 hours of delivery. The mean error for the clinical estimate was 7.9%. The error by ultrasound was 8.2%. There was no significant statistical difference between the two types of estimates, including for the extremes of birth weight.

Journal of Reproductive Medicine, Vol. 33 No. 4, April 1988