Due date statistics: A study on the length of pregnancy

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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 10028 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 10028 records:

Twin births make things slightly out of the norm, so they are not included in the chart. This excludes 89 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 273 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

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Baby age calendar
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holiday shopping lists - kids/babies edition!

Sat Nov 28 4:01 PM by Enigma in Pregnancy & Parenting

My 3 yo nephew wants a pink (or pink and black) watch for Christmas. My sister also threw in a requirement that it not have a character he didn't know on it and he's not a terribly pop cultured kid. It was ridiculous even finding one that met those criteria for his age range and then to try and find a pink watch which won't result in mocking on the playground and at preschool?? OMG difficult. Eventually I found a pink ninja turtles one which I think works. Hopefully he likes it! It really bought home for me the gender divide with kids things. There are so few neutral things in that age range. It's weird how it's actually worse than adults or even older kids.

Generally I aim for a part books, part toy gift for kids on my list. Aiming in the general direction of what I know of their interests.

Childbirth/infant cpr/baby care classes

Mon Nov 2 11:25 PM by antfancier in Pregnancy & Parenting

Thanks everyone! Lots for me to think about...

I have just started prenatal yoga and it seems that it will be pretty useful - although it's way more physical hard work than I expected (I'd better get used to it now, I guess).

Also, I hope I get an inflatable baby from whichever course I end up going with.

Postpartum girdle?

Fri Oct 23 2:47 PM by rowan in Pregnancy & Parenting

Interesting to hear everyone's stories about recovery. I still have issues with some of my ab muscles, I wonder if it would have helped.

OMG, twins! Advice & thoughts?

Mon Oct 19 5:27 AM by BabyTime in Pregnancy & Parenting

Hi :)

I have heard and I have noticed that in twins, one will be very active and one will be dull. Why it is happened with twins? Can anyone give justified answer to my question?

Red Raspberry Leaf capsules - ever helpful for labour or hokum?

Mon Sep 28 3:19 PM by artemiscuous in Pregnancy & Parenting

Oh, this is an interesting discussion and one that I'm finding interesting as I am in the early stages of my second pregnancy.

I really appreciate both perspectives represented here - of modern medicine and of herbalism - and I actually like a middle way of using both, insofar as they are meant to be used (I think both can be and often are misused and misunderstood). I think a well-studied use of herbal remedies can be very complimentary to good medical care. A friend of mine, who is a Western herbalist and clinical researcher, is very evidence-based and really changed my perspective on the place for herbs. It's easy for people to go overboard and claim too many benefits to herbal remedies, to push baseless hokum like homeopathy and chakra balancing, etc., and it's equally easy to denounce it all as unproven, because most clinical studies are not looking at herbs in a double-blind, well-constructed study, particularly in pregnancy, because of the risks if it is somehow dangerous (and many herbs can be, just because they can be quite potent, particularly in a tincture or other concentrated form).

medicated vs. unmedicated birth

Fri Nov 27 2:13 PM by Sonic# in Pregnancy & Parenting

^ How did you get your baby to be a model for an online organic baby care store? (URL in spoilers below)


Letting kids use knives (and other dangerous implements)

Mon Nov 2 8:20 PM by lyra211 in Pregnancy & Parenting

When I was 7 we moved down the street from a 6-year-old boy whose dad owned a woodworking company. Little boy and I became pals, and his dad taught the two of us how to whittle not long after that -- as 6- and 7-year-olds messing with some pretty sharp Swiss Army-type knives, we retained all our digits and had a great time making whirligigs. His dad also taught us other basic woodworking, like hammering and whatnot, and other than a smashed finger or two we were none the worse for wear and I grew up pretty handy with some basic tools. No life-threatening injuries, and I can't even remember any slicing of fingers, although it seems as though there must have been some. I think kids are way more capable than we give them credit for, as long as they're properly instructed and supervised (obviously taking into account some judgment of the maturity level of the particular kid involved). It's hard to imagine a butter knife being a danger, and I can remember being pretty comfortable using my grandmother's steak knives to slice up a baked potato from an even younger age than the whittling.

Glucose Tolerance Test Problems

Tue Oct 20 3:26 AM by Bork in Pregnancy & Parenting

Definitely possible! I mean, the idea behind the tests is the same: measure an individual's reaction to a measured amount of glucose. The biology between GDM and T1DM are different, but at the end of the day I would think that the OGTT (glucose test) results would be similar enough. Although I guess there are so many more hormones in play with pregnancy that the results might end up being pretty different. I find it so disconcerting that doctors are like, "well, this test is somewhat unreliable but you should do xyz because FETUS" in cases like this.

We only tested our kids pre and post drink, since it was hard enough to get them to agree to two blood draws!

Altering my mind about pregnancy

Thu Oct 8 10:58 PM by lovernotafighter in Pregnancy & Parenting

Thank you everyone for invaluable opinions. I'm hoping I will not have to wait till my 40's, but I am 34 now, and the earliest I might have kids is probably 37. It will probably also take longer since I am in a same sex marraige, and ttc and all... Another consideration is that my wife is 13 years younger than I and she will be donating her young and healthy egg (hopefully) to me. So, my genetics will not be the only consideration. All factors I think about, and more... :) Thanks again everyone!

that time I got food poisoning when I was pregnant

Sat Sep 19 12:34 PM by spacefem in Pregnancy & Parenting

We have topics and questions all the time about what to eat when you're pregnant, so I wanted to share this charming story of an experience I had while pregnant with my second daughter.

I was only 10 or 11 weeks along and had been dealing with a "managable" level of morning sickness. This means that I could throw up at any time without warning, but in a cute pregnant way, not a dibilitating one. Most times, as long as there was food on my stomach I'd be okay, so I was careful to keep healthy snacks around - my favorite being cheetos, because you know, cheese is dairy right?

I was following all the good internet advice of not eating raw or uncooked foods, no deli meat, no salad bars, no soft cheeses, only low mercury fish, no food out of trunks of cars, etc.

We went to a wedding at a nice hotel. There was a buffet. I was one of the first people in line at the buffet because I had a two year old who looked like she might be hungry so I played that card, you know how it is. The food was delicious - salad with fresh lettuce, rice, beef strogonoff, mixed veggies. Everything that was supposed to be cold was cold and everything that was supposed to be hot was steaming hot.

Vent, advice: How to not be crazy at work...or in general

Wed Nov 4 3:23 PM by Maro in Pregnancy & Parenting

So, my pregnancy hormones are causing me major mood swings right now. I don't necessarily feel ashamed of it, but I'm frustrated enormously by it because I've got a ton of projects at work and a two-year-old to deal with, and I honestly do not have time to sit and cry for 30 minutes every few hours. I really hate being pregnant, for that and other reasons, but right now it's mostly that.

This morning I felt awesome and totally zen and amazing and only vaguely nauseated, and then a couple of things hit at work that I routinely eat for goddamn BREAKFAST and I'm a puddle of tears in my office. Luckily, I work from home so no one can see me, and all I can really do is push through it and keep getting stuff done. But it majorly sucked. Maybe there's no way around it...but if anyone has any ideas for coping with this stuff I'd be grateful. When you Google pregnancy hormones and mood swings the majority of advice is aimed at husbands "surviving" it, which immediately sends me into a rage spiral.

hospital vs homebirth

Sat Oct 24 8:02 PM by careme777 in Pregnancy & Parenting

The numbers don't lie. Home birth is several times riskier. For myself, being a risk averse type, I wouldn't choose it (even if I lived in a place where midwifery was licensed and part of the health care system.)

I'd be curious to know which numbers you're referring to. I chose to do a home birth (living in Canada, with government funded midwifery care) after a LOT of research (on an unrelated note, we ended up transferring to the hospital for a C-section during the pushing phase because baby wasn't turned correctly).

I read most of skepticalOB while trying to make my decision and did a basic (given my limited access to databases) lit review. My conclusion was that skepticalOB might have a point when discussing home birth in the US, because of the way midwifery care is stratified into certified nurse midwives and lay midwives. In mostly every other developed country with midwifery care, however, there is no conclusive evidence one way or the other that home birth is less safe than hospital birth for women with low risk pregnancies.


Mon Oct 19 1:05 PM by Sonic# in Pregnancy & Parenting

^ Should I stop doing one pro-health activity proven to stymie the spread of disease just because I have another one? It seems inaccurate to use cholera, a disease for which children in Western countries are not usually vaccinated for precisely the reasons you describe, to talk about all vaccinations. A better example is whooping cough, a highly contagious bacterial infection which doesn't need waterborne or foodborne vectors in order to travel and which vaccines prevent.

I don't think anyone here was saying vaccines are the only relevant factor. I heard Spacefem saying they're an important factor because even healthy living and good infrastructure won't prevent the spread of disease, and even good country living (not city living) is no panacea.

A bit aside from the topic, but I'm getting a flu vaccine this year because one of my colleagues cannot receive vaccinations due to a condition that suppresses her immune system. If I said, "Modern society is much cleaner nowadays and we eat a lot better, so I'm going to opt out of the vaccine and expect you to tough it out," I'd expect her to yell at me. Now, some of the hallmarks of that decision differ - I'm an adult who voluntarily takes the vaccine. The impact is similar - I reduce for the more vulnerable people around me (kids, elderly, anyone else with suppressed immune systems) the risks associated with infection.

Real stats for C-Sections?

Sun Oct 4 2:18 AM by Gnarlbanya in Pregnancy & Parenting

Thinking about this topic a little more, I think it's important that we remember that statistics just indicate trends and don't actually tell much of a story as far as individual circumstances go. By that I mean that while in some countries the c-section rate is statistically higher than in other places, it doesn't mean that an individual's decision to have a c-section in those countries is necessarily a flawed one. Nor does it mean that all doctors in those countries are scalpel-happy and just want to get babies out so they can take off to play golf or go for lunch, as I've seen it put on other pregnancy forums (not here).

If I do a brief statistical analysis of friends/acquaintances etc who have given birth in the last few years, the majority had vaginal births, everything from quick and uncomplicated to lengthy and forceps delivery. I can't think of any who had c-sections as a personal choice rather than for a medical reason. Those who had c-sections either had scheduled ones for good reason (obstruction due to fibroids, placenta previa, breech after unsuccessful ECV) or an emergency one for good reason (pre-eclampsia, fetal distress, placental abruption etc). Australia apparently has a c-section rate of 20-25% which this sample would accord with. But given the reasons for the c-sections, I do wonder when I read that the WHO recommends a c-section rate of 10-15%, who exactly should have been turned away from this limited sample?

Prenatal genetic testing

Thu Sep 17 9:08 AM by rowan in Pregnancy & Parenting

MOD HAT: I have moved the vaccination discussion to a new thread. Please discuss vaccines there.