The Lie of the EDD - Why your due date isn't when you think

[LimeSlice]

Please move this if it is in the worng spot...was going to put it in the News Section but it seemed more relevant here


This is from the Associated Content Website: The Lie of the EDD: Why your due date isn't when you think

We have it ingrained in our heads throughout our entire adult lives-pregnancy is 40 weeks. The "due date" we are given at that first prenatal visit is based upon that 40 weeks, and we look forward to it with great anticipation. When we are still pregnant after that magical date, we call ourselves "overdue" and the days seem to drag on like years. The problem with this belief about the 40 week EDD is that it is not based in fact. It is one of many pregnancy and childbirth myths which has wormed its way into the standard of practice over the years-something that is still believed because "that's the way it's always been done".

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

[Ryatha]

I like this article, thanks for finding it Alexis - it supports my decision to get rid of my pregnancy ticker and just enjoy the last few weeks of my pregnancy rather than counting down the days...

[Fleur]

Great article Limeslice, thanks for posting

It's something I'm particularly interested in this time, wanting to avoid induction & with a history of 'overdue' babies...I'm going to try the method here for calculating EDD & see how it pans out....

Can't say I am ready to give up my ticker Ryatha ! although I understand your decision.
I like to see time passing but I'm not as fixated on it this time (as in previous PGs), I'm feeling much more like 'bub will come when she's ready', and I totally hear you about enjoying the last weeks.

DH is getting impatient though. Might show him this

[Indadhanu]

WOW thanks for posting this Alexis!

[tk1999]

Great article LimeSlice...

Very useful for me... lol I am already feeling a need to be prepared for an arguement as to why I should be allowed to 'go over' my EDD if the need should arise... the silly thing is I am not currently being pressured by anyone but I have this niggling feeling that I need to be prepared

[HotI]

It was interesting to read how the 40 weeks measure came about, however the difficulty of varying cycle lenghts in Naegele's Rule still exists with Mittendorf's Measure.

The 'easy' way of calculating EDD using Mittendorf's adds extra error because it does not take into account the varying number of days in a month.

ACOG itself (American College of Obstetricians and Gynecologists) does not recommend interfering with a normal pregnancy before 42 completed weeks.

This was the most relevant part of the article. Replacing one calculation with another doesn't really help. Educated mothers and health care professionals that are willing to listen to mothers and the recommendations of the governing boards (when pregnancies are healthy) is the best way of avoiding unnecessary early intervention.

[KatieRabbit]

Thanks Alexis. It would be great for a mums and dads to be and all involved in childbirth to be aware of this. The 'Oh my EDD has passed, I must get induced etc etc' is unfortunately still something that I comonly hear among friends, rellies and colleagues!

[Neenee Jellybeanie]

The inaccuracy of ultrasound

First trimester: 7 days

14 - 20 weeks: 10 days

21 - 30 weeks: 14 days

31 - 42 weeks: 21 days

What does this mean? Is this the number of days a dating ultrasound during these weeks is likely to be out by?

[Ozziehoffy]

Great article! Wish I'd seen that to forward to my mum when she was hassling me to give her a DATE - I kept saying that it was soon (this was 4 days before he decided to come lol) and when the baby was ready, but she wanted a DATE because the doctor of someone else she knew gave her a date...ummm ok BUT it's only an ESTIMATE!!! So I went FINE it's 18th... lucky for me he decided to come a few days early to save his mummy from being annoyed to death! I've actually always said that pregnancies are in fact an average of 42 weeks long not 40.

[Laranna]

Thanks for sharing LimeSlice. It really aggravated me when I went to my first doc appt and scan knowing that I have a longer cycle and ovulate later, therefore knew when my EDD was likely to be, only to be told by the so called experts that my calculations were wrong and they moved my dates a week forward based on my LMP even though the scan proved otherwise. Thankfully, my OB actually agreed with my dates

When people ask me when am I due I simply say January and they get all funny when I don't give them an exact date, they can't process the idea or something.

[danniellabella]

That makes a lot of sense.

I guess I'm one of the "lucky" ones who did have a 28 day cycle and knew when I ovulated and my ovulation day and EDD based on LMP were only 1 day different and all ultrasounds confirmed the EDD...but then DD came 4 days early!

[~Saram~]

just subscribing so I can find this when I get home from school pick up

[Cassius2]

wow! using the latest research DD was born really early. it kinda makes sense as she was tiny and had breathing issues for the first few weeks...

cool article!

[BeautifulMadness]

Sooo according to that i wasnt overdue lol

[LimeSlice]

s this the number of days a dating ultrasound during these weeks is likely to be out by?

Yes.

[~Saram~]

Thanks Alexis - that was great reading. I was talking to a lady the other week - think she was a midwife or doula and she was say that your due date can also be effected by your ethnicity. Most caucasion pregnancies are 41 weeks while African pregnancies tend to be 38 weeks. Apparently there is also a family somewhere and all the woman in that family have some abnormally short gestation but there babies are born "full term" so for them they have a marked shorter gestation then most people - very interesting!

[Fleur]

Apparently there is also a family somewhere and all the woman in that family have some abnormally short gestation but there babies are born "full term" so for them they have a marked shorter gestation then most people - very interesting!

That IS interesting... I guess it stands to reason that the reverse may be true for others...that some babies take longer to cook....

[squidipa]

that was very interesting with my porridge