Pregnancy Really Isn’t 40 Weeks

Posted on June 13, 2009. Filed under: Pregnancy | Tags: , , |

It’s all a lie! Well, maybe not a lie, but it’s certainly not the truth. We have always heard that pregnancy is nine months. Then we get pregnant and we learn it’s 40 weeks. Really, it’s neither. Due dates are very rough estimates and only 4-5% of babies are born on their due dates (I tried to find an organizational reference on this but couldn’t. There were many other references stating this estimate so I went with it).

So how did it come to be that we count pregnancy as 40 weeks? It’s called Naegle’s Rule. It’s a method used for estimating a woman’s due date that was developed by German obstetrician Franz Naegele. The date is determined by taking the first day of the last menstrual period (LMP), adding seven days, subtracting three months and adding one year. It was originally suggested 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 then publicized in 1812 by Naegle and became the standard calculation. The problem with this calculation is that lunar months are actually 29.53 days so if you count lunar months (which is what the original Biblical reference was doing) then pregnancy is 295 days which is 15 days longer than the what Naegle suggested. His rule is based on what he believed and not empirical data.

There have been many studies on this topic. Most conclude that the average pregnancy, if left alone, will last until 41 weeks 1 day or eight days past the due date.

The early in a pregnancy, the more accurate ultrasound is for determining gestational age.

The inaccuracy of ultrasound

First trimester: 7 days

14 – 20 weeks: 10 days

21 – 30 weeks: 14 days

31 – 42 weeks: 21 days

Another thing worth noting is that ACOG doesn’t recommend inducing labor until at least 42 weeks. This is why it helps to know your conception date, if possible, so that you can arm yourself with the right information if you doctor is threatening induction if you go past 40 weeks.

I know how hard it is to be pregnant past 40 weeks. I went to 41 weeks 1 day myself. It’s HARD. But it’s also better for you baby and your body to allow labor to spontaneously begin on it’s on. We need to forget the whole 40 weeks of pregnancy rule. We need to ignore people who say “you haven’t had that baby yet?”


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12 Responses to “Pregnancy Really Isn’t 40 Weeks”

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Ha ha – so true! This is why I lied to everyone at work about my due date- less pressure!. I knew I’d go over And I did- 42 weeks exactly before water broke. My doc was great and was still in no hurry to induce at 41 wks 5 days. Unfortunately, she didn’t deliver me and I ended up with a csection.

That was probably smart thinking to tell everyone the wrong date! It is so stressful having that number.

I am “39” weeks and at my last appointment I was told that they wouldn’t let me go past 41 weeks because the placenta does work as well. I do not want to be induced and I think there is a chance my due date is wrong. Can your doctor actually make you get an induction? Can’t I just refuse it if the baby is fine?

Hi Lanae,

I am sorry to hear about the pressure the doc is giving you concerning your due date. Technically they can’t make you induce, I mean, holding you down and forcing you against your will is kind of illegal 🙂 They can give you a very hard time and use scare tactics and such that would make it very difficult for you to deny the induction. I don’t particularly like the ACOG pamphlet that discusses this but here is the link so that you can at least quote ACOG as ammo i f you need to buy some time…

Basically it discusses the same thing your doc is saying but at 42 weeks rather than 41 weeks. There are quite a few docs using 41 weeks these days and it drives me a little nuts. My suggestion is that if you don’t have the baby by 41, have an NST and if all is well stall. Even if you go to 42 weeks it doesn’t’ mean that disaster is on the horizon. MANY many women go past 42 weeks and even 43 weeks and it’s fine. Some people are long gestatators and the estimated due date is just that, an estimate. I’d follow my instinct, listen to my body and baby and stick to my guns. My own doc agreed not to even mention the word induction until I hit 42 weeks at which point we would decide then what the best course of action was based on NST results and Bishop’s score (induction success estimator).

Good luck and keep us posted!


Yah it caught me off guard when they mentioned it at my last appointment. I asked why they don’t wait til 42 weeks, although I did see a student so I’m not sure if that had anything to do with it. I’ll definitely talk with the doctor on Friday. They said if I’m still pregnant by then that they will schedule an ultrasound to make sure my fluids are okay ad that they baby is okay. So I figure if everything is fine that we should just wait. I have no desire to be induced, I don’t want to be given pitocin or any of the other drugs they use. thank you for the information I’ll have to ask them about my Bishop’s score.

Great post!! Thank you for the information. It will be great to share with my doula clients.

You had me until the end, when you talk about ARMING yourself with info in case the doctor THREATENS induction. There are docs and midwives out there who might be waging war on their patients/clients, but most medical professionals are basing their decisions on their expertise and a desire for the best outcome for mother and child.

We have to be careful about the language we use. When I was expecting my first, I was so susceptible to the idea that I was going to have to fight and that all medical professionals were there to cajole and intimidate me…it created s defensiveness that was totally unnecessary.

My advice to anyone who is seeking care during pregnancy and birth is to have an open mind to their care provider’s reasoning, and to ask a lot of questions to clarify the information you already have from your personal research. Docs can be very open. If your care provider is the type that is not receptive to questions, do your best to find another!

(Sorry for the TL; DR…)
One last thing, according to ACOG, gestation is *between* 40-42 weeks. Because risks increase from around 40 weeks, depending on other factors there may be good reasons to induce. Each situation is different, and each woman has to decide for herself what her comfort level is with those risks.

Hi Miriam,

When there are hospitals that have 80%+ induction rates and 50%+ c/s rates, you need to be armed with information. Also, ACOG recommends induction for post dates which is 42 weeks. In the absence of other medical indications elective inductions are not advised.

ACOG’s guidelines, like you mentioned, define 42 weeks and beyond to be a postdate pregnancy. But they actually advise three options during the “impending postdate pregnancy”, period which is considered to be 39-42 weeks: elective induction, expentant management, or antenatal testing. They approve of any of those options. Elective induction requires documentation of a positive pregnancy test from at least 36 weeks earlier (decreasing likelihood of misdated gestational age). Risks do actually increase beyond 39 weeks (meconium aspiration and stillbirth, i.e.), but of course there can be risks associated with induction (increased risk of cesarean).

Here’s a great source for what I’m referencing:

Thanks for writing this post! I still stand by what I said about being cautious with the language you use when framing the doctor/patient relationship as antagonistic, but I understand your point that women need to have the correct information to avoid unnecessary intervention.


words:you’re using them wrong

[…] pregnancy, if left alone, will last until 41 weeks 1 day or eight days past the due date."…isnt-40-weeks/ For those who have had babies is this a more accurate time frame?I know that with my mum she had […]

like your article. ovulation is key to conception. now researchers have found out that women may vary in their length of pregnancy.

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