The Lie of the EDD: Why Your Due Date Isn't when You Think
The folly of Naegele's Rule
The 40 week due date is based upon Naegele's Rule. This theory was originated by Harmanni Boerhaave, a botanist who in 1744 came up with a method of calculating the EDD based upon evidence in the Bible that human gestation lasts approximately 10 lunar months. The formula was publicized around 1812 by German obstetrician Franz Naegele and since has become the accepted norm for calculating the due date. There is one glaring flaw in Naegele's rule. Strictly speaking, a lunar (or synodic - from new moon to new moon) month is actually 29.53 days, which makes 10 lunar months roughly 295 days, a full 15 days longer than the 280 days gestation we've been lead to believe is average. In fact, if left alone, 50-80% of mothers will gestate beyond 40 weeks.
Variants in cycle length
Aside from the gross miscalculation of the lunar due date, there is another common problem associated with formulating a woman's EDD: most methods of calculating gestational length are based upon a 28 day cycle. Not all women have a 28 day cycle; some are longer, some are shorter, and even those with a 28 day cycle do not always ovulate right on day 14. If a woman has a cycle which is significantly longer than 28 days and the baby is forced out too soon because her due date is calculated according to her LMP (last menstrual period), this can result in a premature baby with potential health problems at birth.
The inaccuracy of ultrasound
First trimester: 7 days
14 - 20 weeks: 10 days
21 - 30 weeks: 14 days
31 - 42 weeks: 21 days
Calculating an accurate EDD
Recent research offers a more accurate method of approximating gestational length. In 1990 Mittendorf et Al. undertook a study to calculate the average length of uncomplicated human pregnancy. They found that for first time mothers (nulliparas) pregnancy lasted an average of 288 days (41 weeks 1 day). For multiparas, mothers who had previously given birth, the average gestational length was 283 days or 40 weeks 3 days. To easily calculate this EDD formula, a nullipara would take the LMP, subtract 3 months, then add 15 days. Multiparas start with LMP, subtract 3 months and add 10 days. The best way to determine an accurate due date, no matter which method you use, is to chart your cycles so that you know what day you ovulate. There are online programs available for this purpose (refer to links in resources section). Complete classes on tracking your cycle are also available through the Couple to Couple League.
ACOG and postdates
One of the most vital pieces of information to know when you are expecting is that ACOG itself (American College of Obstetricians and Gynecologists) does not recommend interfering with a normal pregnancy before 42 completed weeks. This is why knowing your true conception date and EDD is very important; if you come under pressure from a care provider to deliver at a certain point, you can be armed with ACOG's official recommendations as well as your own exact due date. This can help you and your baby avoid much unnecessary trauma throughout the labor and delivery. Remember, babies can't read calendars; they come on their own time and almost always without complication when left alone to be born when they are truly ready.
Sources:
Mittendorf, R. et al., "The length of uncomplicated human gestation," OB/GYN, Vol. 75, No., 6 June, 1990, pp. 907-932.
ACOG Practice Bulletin #55: Clinical Management of Post-term Pregnancy
Published by Misha Safranski
Ms. Safranski is a freelance writer specializing in fetal/maternal safety, VBAC advocacy, and cesarean prevention issues, and also holds a position in Title Quality Assurance with Demand Media Studios. View profile
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37 Comments
Post a CommentThis may have already been posted but the website you have marked as a "resource" is incorrect it should be www.ccli.org. Thanks for the good info.
Hey - ran across this and thought it was really interesting, so I looked up that study you talked about (the Mittendorf one, available here: http://journals.lww.com/greenjournal/toc/1990/06000 ) but I'm not sure it's something we should use to start recalculating everybody's due dates. It wasn't like a large-scale thing, the recommend further research. Their study consisted of only 83 women who'd had previous births, and 31 first time mothers.
And when I looked at other surveys I found some that used thousands of births... one that used the entire population of Sweden and found 281 days as the mean, a Nguyen/Larsen/Engholm 1995 study of 17,000 that found 282 days, a 1965 study of 1700 that found 279 days... these all seem to tell the story that 280 days is a pretty good average.
Now, I won't deny that being "overdue" is a little misleading. If the average pregnancy is 40 weeks, that means 40 weeks is just a center point... most births fall on
I respect that people are being induced far too often and for the wrong reasons but i just had my 3rd baby (with the aid of induction. I was induced at 6 days past my due date and she was almost 11 pounds. She had already passed her meconium and i dread what could have happened had i waited any longer. She could have gone into distress and then been too big for me to deliver naturally! My second had been induced at 8 days late and was red and wrinkled...and over 9 pounds herself...very ready to come out! I'm sure lots of babies shouldn't be induced but there are lots that should be to not have health problems for the mother and baby!
Worries me that everyone will now refuse induction when over their 'dates'. Overdue can lead to placental insufficiency and HIE (brain damage due to lack of oxygen). Also increased size leading to increased possibilty of shoulder distocia and HIE.
Dont mean to worry anyone all I'm trying to say is, If your midwife/doctor is worried about your baby being inside for too long, please listen to their reasoning and don't presume they are just sticking to the rule book x
Our next visit baby turned no problems (Frank Breech would not) I called it a Midwifery two step. 8 times nearer term (17 days over according to hospital) there were 7 attempts to book my daughter in for induction . She refused - baby born later normally and well at the age of 9 she is in the bright class category and reads 2 books at a time.
So glad someone wrote this up I keep rabbiting on about this I am old enough as a midwife to have been with women well short of 40 weeks and well past 43 weeks. Part of Naegle's rule is that 37 weeks + 43 weeks = 80 divide by 2 and you get an arbitrary figure. of 40. My daughter had 6 week cycles- so at 32 weeks our calculation the hospital person with ultrasound stated that she was a breech ("Frank")= legs straight up beside the head breech) and 35 weeks my midwife hands felt a well flexed baby. (50 years clinical and Senior Academic Lecturer of recent f15 years) Babies according to the "rules" do not turn usually until about 32 - 34 weeks. I tried to explain the conception times related to a 6 week cycle. No, Off to Anesthetist for appointment hospital rule 1st baby breech must have a Caesarian. My home birth friend sent a letter on the "how to of turning" to my daughter (as a mother I was not being heard by daughter either). Our next visit baby
Hi J - thanks for the compliment. :) The change to 'Congress' was made after I wrote this article. :)
Great article - very informative! I did see one error. You refer to ACOG as the American College of Obstetricians and Gynecologists, but it's actually "Congress", not "College".
Thank you for that interesting insight, Luna - good stuff. :)
Onanisland - the issue is that the first day of last period is an inaccurate method of estimating the EDD. Every woman's cycle is different, making conception dates very different. Some babies are born at 36 weeks, some at 44, and it's all 'normal'. While ACOG does classify 42 weeks as within the range of normal, how many women do you know who are freaking out at 40 weeks because they are 'overdue'?
I have never heard anything else... a baby will be born between 38 and 42 weeks of pregnancy, calculated from 1st day of last period. Not much new in this article.
@ canadian mom; aren't you looking at al doctors and midwives as individuals :)
If all goes well, a healthy baby is born and everybody is glad and proud of mother and child. If anything goes wrong, everybody is mad at the doctor and midwife...