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parents and babies

Due Date Estimation Fallacies

The following is a list of recent students’ baby’s birth weights alongside their arrival pre- or post- the EDD (“estimated due date”).

These numbers validate the 2 week margin of error with the “due date” women are given.

The weight tells us more about the readiness of the baby.

Look at the weight relative to the “due date.”

These due dates were calculated by the commonly used Naegele’s Rule. (1820s)  It is believed by some that Naegell intended for the count to be from the LAST day of the LMP, not the first.

Midwife Carol Woods Nichols 1980s research produced what we call Nichol’s Rule (for estimating due date), which adds about 5-8 days to the commonly used Naegele’s Rule, bringing it more into symmetry with the theory that Naegele intended the count to be from the last day of LMP.

To estimate your due date with this more accurate method, simply add 5-8 days to your current due date. This is now when women are being induced, instead of starting our 2 week margin of error at this point. Since we don’t TRULY KNOW the gestational age of the baby, and we are using Naegele’s Rule, many babies are being induced and forced to be born, possibly weeks before they are ready. And women’s bodies are being forced into labor (often ending in cesarean) when they are not ready (hormones, receptors to hormones, pliable bones, etc.)

Using Nichol’s Rule, we begin evaluation of the baby at 41.5 weeks.

We evaluate it’s condition, rather than looking at dates only, because the incidence of “postmaturity” (infant is compromised) is very low, even in truly postdates pregnancies.

Birthweight alongside Due Date vs. Birth Date

55 births using Naegele’s Rule

21 before DD

5 on DD

29 after DD

7. 5     13 days PRE

6.15    6 days PRE

6.11    6 days PRE

6.15     8 days PRE

7.14     1 day PRE

7.7       1 day PRE

6.5       3 days PRE

7          7 days PRE    

6.7       11 days PRE

8.4       2 days PRE

7.12     5 days PRE

6.3       5 days PRE

7.3       6 days PRE (induced, predicted 8.13)

5          10 days PRE

5.4       17 days PRE

7          2 days PRE

7.14     4 days PRE

7.4       6 days PRE

8.2       1 day PRE

5.6       13 days PRE

7.14     3 days PRE

7.10    3 days POST

6.7       3 days POST

8.5       1 day POST

6.2       11 days POST

7.6      12 days POST

8.5       14 days POST

7.5       2 days POST

6.4       8 days POST

8.12     10 days POST

7.4       9 days POST

7.8       4 days POST

7.9       10 days POST

8.11     1 day POST

8.14     5 days POST

9.5       14 days POST

8.11     6 days POST

8.6       3 days POST

7          5 days POST

8.4       7 days POST

7.1       12 days POST

7          9 days POST  (IVF)

7.1       4 days POST

6.15     13 days POST

7          4 days POST

7.13     7 days POST

7.15     14 days POST

7.12     6 days POST

7.10     5 days POST

7.3       7 days POST

7.6       Due Date

6.8       Due Date

7.2       Due date

9          Due date

5.9       Due date

“please let me be born when I’m ready!”

Summary of Nichol’s Research

Journal of Nurse-Midwifery Volume 30, Issue 5, September 1985/October 1985, Pages 259-268

Postdate pregnancy Part II. Clinical implications

  • Nichols, C.W.
  • Maternal-Newborn Nursing/ Nurse-Midwifery Program, United States


This is the second part of a paper on management of the postdate pregnancy. The first part, a comprehensive review of the literature, forms the foundation for this clinical research. Data from 175 consecutive deliveries with the Yale Nurse-Midwifery Practice are presented, with specific emphasis on gestational age and infant outcome measures. Postdate pregnancy (exceeding 42 weeks’ menstrual age) occurred in 10.3%, while the incidence of postmaturity was 1.7%. Certain complications such as fetal distress and meconium aspiration are significantly more common for the postdate infant, and it is more likely that the postdate gravida will have experienced prenatal bleeding and will need oxytocin stimulation in labor. A management protocol of fetal evaluation is proposed. At 41.5 weeks, daily fetal movement assessment is begun. At 42 weeks, a nipple stimulation contraction stress test and an ultrasonic biophysical profile are suggested. Routine labor induction on the basis of dates alone is discouraged.