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Due date statistics: A study on the length of pregnancy

[ Survey Results Intro | All Charts | Take the survey ]

Do babies come on their due dates?

This was the question on my mind during the later stages on my pregnancy in the spring of 2010. I heard a lot of conflicting information about what a "due date" meant and when babies were born. Some people saw the "due date" like a deadline... if the baby hasn't arrived yet, it's late, it's time to freak out. Some people said due dates were a bad example of western intervention, used by doctors as an excuse to cut us all open. Some people said that first time moms should expect to go late, it's normal and you should chill out. I found some scattered statistics about what week babies came... and a lot of those graphs just said "X% were born at 40 weeks" but didn't explain what that meant (during the 40th week of pregnancy? 40 weeks accomplished? On day 280?). And trust me if you're pregnant, you start counting DAYS when that time comes around. There is a huge difference between 40 weeks 0 days and 40 weeks 5 days.

I was unable to find great data that satisfied me. So, armed, with a website, I started running a simple survey about due dates. If you've had a baby, please take it!

So far the survey has had 8091 participants, and I've put together these pages of charts that show interesting things I've learned from the results. I am not a mathematician, so there's no in-depth statistical analysis. I'm an engineer, web programmer, and (as of 2010) mom. When I started the survey I was just a very very pregnant woman wondering when my baby was going to show up.

This first intro page is all about the methods used and why I think it's valid. If you don't care about the background, just want to see the dang averages, skip to this page or pick one of these:

Due date statistics: A study on the length of pregnancy
Probability of delivery resulting from spontaneous labor after 35 weeks
Probability of delivery within x days of a given date
Length of pregnancy by week
Spontaneous labor and due date determination
Length of pregnancy, comparing subsequent births for individual moms
Length of pregnancy for first time vs. second & third time moms
Length of pregnancy, type of delivery
Gestation vs. Birthweight
Probablity of Induction after a given day
Average day of spontaneous labor vs. age of mother at time of birth
Are more babies born during a full moon?
What's the most common day of the week for babies to be born?
I'm still pregnant at 40W. What's it mean?
Do winter babies arrive later?
What if I know my conception date?
Are boys or girls born later?
Subsequent pregnancy date search
How do inductions bias the due date statistics?
Birth Stories
Survey input dates

For reference, here's the very basic plot of who took the survey:

Other data

I was unable to find daily breakdowns of when births occur, but there have been other studies about due date accuracy. I think the data is hard to find because the medical community has accepted the fact that it's impossible to exactly predict when a baby will show up, so they're okay with just knowing about when it happens, and us pregnant women are also urged to be okay with just knowing about when it will happen. But if you're a geek like me who loves statistics, that's tough.

All these studies describe the length of pregnancy in days. For reference, 280 days = 40 weeks = 9 months 6 days.

The length of human pregnancy as calculated by ultrasonographic measurement of the fetal biparietal diameter (H. Kieler, O. Axelsson, S. Nilsson, U. Waldenströ) has a very nice table of 10 published studies, which found various means between 272-283 days. The authors contribute their own results as well, recording over 800 mothers who went into labor spontaneously. When pregnancy length was calculated using ultrasound in the second trimester the mean was 280.6 days, standard deviation 9.7 days. When it was calculated using LMP, the mean was 283.6 days with a standard deviation of 10.5 days.

A study from the Swedish birth registry involving 427,582 singleton pregnancies the mean, median and modal durations of pregnancies were 281, 282 and 283 days respectively. The standard deviation was 13 days. This study did not consider whether labor was medically induced or not. (Duration of human singleton pregnancy—a population-based study, Bergsjφ P, Denman DW, Hoffman HJ, Meirik O.)

A 1999 study of over 17,000 spontaneous deliveries stated that due dates are more accurate if we add 282 days to LMP instead of 280 (Evaluation of ultrasound-estimated date of delivery in 17,450 spontaneous singleton births: do we need to modify Naegele's rule? Nguyen TH, Larsen T, Engholm G, Møller H.)

A 2003 London-based study of over 122,000 pregnancies with spontaneous onset of labor (except they call it "labour" because, well, you know) found that average gestational age at delivery was 40 weeks for white Europeans but only 39 weeks for members of the black and asian ethnic groups. I chose not to ask for ethnicity or race in my survey because it's so hard to define, so many people are a combination of races, but the study is interesting. Another note: it was made up entirely of first time ("nulliparous") mothers. (International Journal of Epidemiology 2003;33:107–113 Roshni R Patel,Philip Steer,Pat Doyle,Mark P Littleand Paul Elliott)

And finally there's this 1990 Harvard study called The length of uncomplicated human gestation (Mittendorf et al). The study looked at 31 first-time moms at one private practice to conclude that their average due date was eight days early. That's not a very big sample size, which I think is why it's the only study that found 288 days as the median. I really wish people would stop bringing it up, but it seems to be popular so I'll just say if you want to read it and come to your own conclusions, it's here.

Come on people, if 288 days was really the median that means that more than half of all pregnancies would end past 41 weeks... seems like we would have noticed something like that by now.

There is good weekly data from the CDC that talks about birthweight and weight gain and inductions and all kinds of good stuff. But it's not the daily breakdown I wanted, and it doesn't break down weekly stats by whether the births were spontaneous or induced.

Does your survey consider age, race, cycle length, shoe sizes, ferret ownership, etc?

It became apparent to me by reading studies that no matter what factor people added into consideration, it was still impossible to predict the start of labor. Even IVF patients, who know the HOUR their baby was created, don't get accurate due dates. So I see no point in taking all these factors into consideration, if it's going to be a range of dates lets just all throw whatever randomness we have into the pot and see what the range is. Several studies and my survey have found that there's a standard deviation of over a week... this makes the bell curve pretty flat on top. Moving a due date a day up or back based on ovulation really makes no difference because all the dates around 40W are almost equal in terms of your probability of going into labor.

You realize that an internet survey isn't exactly scientific, right? It's going to be so biased!

Science wasn't really my goal. By that I mean I'm not terribly interested in how the human body works... I'm interested in how life is for women like me. The women who are reading this and visiting my website are mostly internet users from english-speaking countries... well, so are my survey participants. I see that as a good thing.

That said, even if it's not scientific the fact that the results are matching real scientific studies mean that I must be doing something right. The studies mentioned a few paragraphs above here seem to support the fact that the 280 day mean, 9-10 day standard deviation is the norm... well that's what my results show too. I think if the big numbers about the curves are correct than the little details are likely to be correct also.

Where does "40 weeks" come from?

The most common way to calculate a due date is to set it at 40 weeks (280 days) past the woman's last menstrual period. This is known as Naegele's Rule, after a German doctor who published the method in 1806. Naegele did not arrive at this method very scientifically, but I personally believe that we would not be using it if it was totally off.

Did you consider that augmenting labor can change the birth date?

No. Because I don't care that much. Most augmentations can only change things by one day, if that. I also feel like I would have had to consider the whole range of things... there's the doctor that starts Pitocin when a woman is at 3cm and there's the doctor who's completely hands off until reaching for the foreceps at the end. Both of them decreased labor time and might have change the day a baby was born, but to very different degrees. And anyway, my goal was to tell women who know nothing when they might have a baby, since most of these dates are totally a surprise. When you're very pregnant, there's no way to tell when labor will begin and you go looking for internet charts. When you're in active labor, there aren't as many questions in your mind.

Who cares what's normal? Shouldn't we just tell ourselves to accept whatever happens or be patient?

Yes and no.

On one hand, I think it's a good idea to spread the word that pregnancy does indeed usually last 40 weeks, and if a woman is 37 weeks pregnant it's not only scientifically inaccurate to tell her she's "about to pop", it's also really annoying.

On the other hand, I've seen this survey discussed with comments like, "Well obviously a bunch of women had babies past 42 weeks, this shows that we should just be happy being pregnant forever, fight those inductions, trust nature." That's a risky assessment. There is significant evidence that health risks to babies increase as pregnancy approaches 42 weeks (Fetal and perinatal mortality, United States, 2005 by MacDorman et al. is one from the CDC). I think nature is sort of a good thing to trust. Yes, it got the human race this far. Yes, I had a natural birth and recommend it. But for thousands of years 1 in 10 births didn't end well... nature was happy with the 90% batting average. But we can do better. So from that standpoint I'd prefer that you avoid using this study to tell your doctor that you intend to be 43 weeks pregnant. Deal?

What results are included in the charts?

I decided to only include births between & including days 245 (35W,0D) and 301 (43W,0D). This means that out of the 8091 records:

Twin births make things slightly out of the norm, so they are not included in the chart. This excludes 75 results. I didn't really ask good survey questions for twins anyway. If you are pregnant with twins and looking for data, there's a good survey going over here.

In addition, around 200 results I started noticing a big spike in the number of babies born on their due date. I believe this is because women who have their babies on their due dates are more likely to remember it, and be enthusiastic about taking a survey. To account for this I've capped the number of results shown at exactly 40 W, and 234 results are excluded.

Due Date Survey Data

Due date statistics: A study on the length of pregnancy
Probability of delivery resulting from spontaneous labor after 35 weeks
Probability of delivery within x days of a given date
Length of pregnancy by week
Spontaneous labor and due date determination
Length of pregnancy, comparing subsequent births for individual moms
Length of pregnancy for first time vs. second & third time moms
Length of pregnancy, type of delivery
Gestation vs. Birthweight
Probablity of Induction after a given day
Average day of spontaneous labor vs. age of mother at time of birth
Are more babies born during a full moon?
What's the most common day of the week for babies to be born?
I'm still pregnant at 40W. What's it mean?
Do winter babies arrive later?
What if I know my conception date?
Are boys or girls born later?
Subsequent pregnancy date search
How do inductions bias the due date statistics?
Birth Stories
Survey input dates

More Resources

Pregnancy Day-by-Day
Fast pregnancy calendar
Baby age calendar
EDD Icon Generator
Jo's Birth Story
Weeks vs. Months Explained
Charts generated by flot
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Recent Comments

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you are invited to join our pregnancy forum. We are dedicated to answering complicated questions as logically as we possibly can.

How is pregnancy different from when it's shown in TV?

Sat Dec 20 4:29 PM by Zoraptera in Pregnancy & Parenting

The second someone starts feeling contractions on TV, they have to be raced to the hospital immediately! You almost never see people saying, "Okay, let's get out the timing app!" and waiting until the contractions are five minutes apart.

And so many people end up being delivered by a random dude they know. There's an episode of Fresh Prince where Will delivers a baby. In Star Trek:TNG, Worf delivers Keiko O'Brien's baby. And on and on...


Visitors soon after birth

Sat Dec 13 6:21 PM by monk in Pregnancy & Parenting

My sister and her husband were glad for mine and our mothers help for limited periods when she just wanted to sleep but we both understood "the look" that said we should go away without her having to actually throw us out. If you don't have that relationship with "S" and can't establish it before the babies born don't guest her at your house.

But it is a really kind offer. Maybe she doesn't know you already have a support system? let her know.


Confessions of an Evil Stepmother

Thu Dec 11 1:15 PM by Nachos in Pregnancy & Parenting

I have no help, but I can offer hugs. *hugs*


Finding out baby's sex before birth

Wed Dec 3 3:54 AM by aka_laura in Pregnancy & Parenting

We found out, it was a fun detail to learn about our baby, it inspired the Wonder Woman nursery theme. My friends and family didn't go too nuts on the pink. There were a number of weird conversations with people who assumed I would oppose pink. I think pink is a lovely color for a baby to wear, alongside all the others, but people associated feminism with some kind of anti-pink venom that I just don't feel. I guess I feel like eventually, my family and friends are going to know the kid's sex. Our culture will ascribe gender roles to her far before she's conscious of them herself, so she'll have to be taught from a very young age that such things are bullshit. Wether she starts being gendered at 3 months or 3 days doesn't stand to change things significantly, in my view, we still have a long road on that one.


Pregnancy Books

Sun Nov 30 2:38 PM by Metcodon1 in Pregnancy & Parenting

For a good pre-conception book with a lot of science in it, I really enjoyed "The Impatient Woman's Guide to Getting Pregnant". All the science is cited and it's an easy read.

I really liked the book "Origins: How the Nine Months Before Birth Shape the Rest of Our Lives". It contained a lot of information a lot of other books didn't and was fun to read.

For a good reference book, I love the 411 series (Expecting 411, Baby 411, Toddler 411). It has really helpful information in an easy to look up format.

I also have enjoyed reading some essays about motherhood including "Waiting for Birdy" about a woman's second pregnancy while raising a toddler, "The Good Mother Myth: Redefining Motherhood to Fit Reality" which is a collection of essays about motherhood, and "Bringing Up Bebe" for a travelogue while parenting.


Keeping calm in new pregnancy after a loss

Sun Dec 14 4:25 AM by ladyhawk in Pregnancy & Parenting

I haven't had to go through anything that is this (I imagine) traumatic. However, I have been pregnant five times, and I do only have two children. During my two successful pregnancies (I miscarried very early in the other three) I pretty much had to just stop reading anything at all, because, for me personally, it wasn't reassuring. It just made it worse. No news, baby books, articles, no online communities, nothing. Just made me more paranoid.

After the 3rd miscarriage, I went to grief counseling, and I continued therapy through my pregnancy with my second child, which seemed to help a lot.

I will tell you, if it gives you any peace of mind whatsoever, that I was told after my second miscarriage, when I was only 20 years old, that I would need a hysterectomy and probably never have children. Five years later, I have all my reproductive organs and have two healthy children.

I know what works for me won't for everyone, but you could maybe try some of these things?


First trimester nausea and keeping it together at work

Sat Dec 13 3:00 PM by aka_laura in Pregnancy & Parenting

Ladies, I am so sorry you're feeling so awful. Do talk to your doctors about this, and expect them to take is seriously. I delayed getting help for way too long because I thought I was just having normal morning sickness and it was NOT normal morning sickness. Once you've tried the usual round of crackers, ginger everything, sea bands, teas, etc. there are prescriptions that can help you stay healthy so you can nurture your growing baby. I had to take enough Zofran and Phenergan to put a horse to sleep and had a healthy, happy baby born to a healthy, happy momma. If things really persist check out http://beyondmorningsickness.com for more options.


The madness of modern parenting

Sat Dec 6 2:27 PM by spacefem in Pregnancy & Parenting

kaybee that is a great write-up, thank you for sharing! do you mind if I split this off to its own topic? because it's worth it!

I especially admired the conclusion:

There is no broad responsibility to create a healthy environment for children (because mothers who were concerned would live in some other environment), and no social imperative to look after children who were born in ill-health or some other misfortune (because mothers who behaved responsibly would have prevented this outcome).


Food restrictions in pregnancy

Wed Dec 3 3:44 AM by aka_laura in Pregnancy & Parenting

Pretty much the only helpful comment I got from a member of the general public while I was pregnant was this super sweet woman at a lecture. We were chatting, I told her I was expecting my first baby, and a few minutes later I mentioned my friends and I had just gone out to a local sushi place. She got this big smile and said "I didn't have sushi until my third kid, GOOD FOR YOU! You'll be a great mom!"

Bless that wonderful woman.


favorite movie about anyone having a baby

Sat Nov 29 4:31 AM by Sonic# in Pregnancy & Parenting

When I was little I was wild about Look Who's Talking. I think the novelty of a narrating baby wore off sometime after I saw Milo and Otis.


pregnancy and anxiety

Sat Dec 13 6:54 PM by Wilde83 in Pregnancy & Parenting

This is my second pregnancy, with unfortunately no children. This statistics page has given me comfort both times even though no one could have predicted the oddball birth defect that ended my last pregnancy in the second trimester. I suppose I was just one of the unlucky 1% last time, and am so hopeful this time. Thank you for posting it!


fired pregnant worker takes it to the supreme court

Thu Dec 11 7:03 PM by jen4knits in Pregnancy & Parenting

The place I work at did the same thing to a temporary type employee when she was pregnant. She had let them know when she was hired that she was pregnant and would be needing a c-secition. But then when the time came she hadn't been at the job for a full year so they said she didn't qualify for FMLA (family medical leave something) and emailed her... (email yes) to say her options were: 1) resign and reapply or 2) call off and get fired after missing work for 4 days.

Needless to say it worries me about my options even though I'm in a different department, with different job type. It will be interesting to see what the ruling is for this and how it affects current policy.


pin it: pocket guide to fish

Sat Dec 6 10:27 AM by spacefem in Pregnancy & Parenting

I have a whole big pregnancy board on pinterest that's kinda fun. There's an infographic version of my due date statistics that a lot of folks have appreciated. So today I added this pocket guide to fish recommendations:

http://www.pinterest.com/pin/135671007500096368/

Totally grabbed from other sources (http://www.fda.gov/Food/FoodborneIllnes ... 393070.htm & http://www.nrdc.org/oceans/seafoodguide/page4.asp) but it mimics a little card that I literally printed out and folded up in my wallet while I was pregnant.


Babyproofing the House

Mon Dec 1 4:16 AM by MFS in Pregnancy & Parenting

We locked down the universe with the first, still pretty tight with the 2nd, but by the time we had #3 and 4, things got a lot looser. We also had so much more stuff that certain areas that were "dangerous" were now totally inaccessible and irrelevant (yay piles of blankies?)


going public with the new pregnancy & parenting forum

Fri Nov 28 7:03 PM by rowan in Pregnancy & Parenting

OMG AKA_LAURA CONGRATULATIONS!