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 9745 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 9745 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 281 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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Recent Comments

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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?

hospital vs homebirth

Sat Oct 3 12:20 AM by Aum in Pregnancy & Parenting

^ I think in the greater diaspora of documentaries on birthing styles, Business of Being Born is pretty good. But I understand your frustration, being in the U.S. The free market there kind of lets people do what they want in a lot of ways. The system in Canada is different. Midwifery is a university program, a bachelor's degree, usually as an attachment to nursing schools. They get trained in modern medicine and can integrate their lineage birthing traditions with modern cutting edge understandings. It doesn't have to be either/or. Kind of disturbing to hear that people in the U.S. can claim to be midwives with a high school diploma. That sounds like a recipe for disaster. I would hope that most people are interrogating the qualifications of their midwife before they commit.

My sister had her first kid at home, the second at a hospital. Both were appropriate, in different contexts. She said that the hospital offered her different methods and she refused them all. Her baby was breech so she had to be in the hospital, but in the end the baby turned and everything happened naturally.

Letting kids use knives (and other dangerous implements)

Mon Sep 28 8:51 PM by ablotial in Pregnancy & Parenting

We didn't have knives at home until I was ... 9 or 10? But they gave us butter knives with school lunch right from the get go, so ... yeah. I never complained about this at home though because I was lazy and liked to be pampered and have my mom cut up my food and spread the butter for me :P

And I did have trouble when they finally gave me real (actually sharp) knives. Steak knives were fine because they were serrated, but I had trouble with paring knives telling which side was the sharp side and often used them upside down trying to saw through something with the dull edge. I finally learned to look closely and learn the difference when my dad got frustrated with how slow it was going for me and slammed his hand down on the knife to push it through (a carrot? celery? can't remember now) and almost cut off his thumb :/

What do you wish you had known?

Fri Sep 18 3:14 PM by rowan in Pregnancy & Parenting

Hm. I dunno, I don't really feel quite as terrified about things as I did for the first several years. Which isn't to say there aren't moments...

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.


Sun Oct 4 12:48 AM by Aum in Pregnancy & Parenting

Aum, you claimed earlier that your medical practice was incapable of doing harm, but please be aware that your medical advice to first put off vaccinating and ignore vaccination schedules and then to be lax about enforcing herd immunity/limiting contact with potential disease vectors actually has the potential to cause a great deal of harm.

My practice has never harmed anybody.

This is just a discussion, I'm not giving advice. I didn't even share my medical title or real name here. Nice attempt at trying to shame me. It won't work though.

It is not as rare as you'd think. Also, although you become MORE contagious once you become symptomatic with pertussis, you can still be very contagious before your symptoms appear. My turn for a personal story:
I caught pertussis as an adult. I had my vaccinations, which gave me the benefit of having a much milder case that I otherwise would have and removed my risk from all the serious complications that can arise from the disease. It still really sucked though, and it was more than three months before I returned to any semblance of normal health. I caught it while working with a group of adults from a religion that does not believe in vaccination. None of them were symptomatic until 36-48 hours after I had worked with them. They spread it to quite a few people during this time. It was determined to be pertussis rather quickly once their symptoms appeared, because most of the unvaccinated individuals ended up in the hospital with severe complications. They also spread it to a few babies, all of which ended up with extended stays in the hospital. Ever seen a baby with whooping cough? I'll spare you the graphic details, but be assured that it is extremely frightening for everyone involved. One of the babies ended up with chronic emphysema as a result of the disease; the others recovered fully after their hospital stay. Either way, such precautions aren't paranoia; they are simple steps people can take to help prevent disease. This is especially important for parents taking your advice on delaying vaccinations.

medicated vs. unmedicated birth

Fri Oct 2 10:46 PM by spacefem in Pregnancy & Parenting

This came up a little bit in some topics but I figured I'd make an on-purpose topic on it, because I have some experience, on both sides interestingly enough.

I've now had two babies, one without any medication in labor and one with an epidural. And I can honestly say that if were to go for a third (not in the plans, but hey) I... have no idea what I'd do. So that's my answer for the perfect birth plan: whatever.

Here are some things I do know:

Those natural birth techniques really do help with managing painful contractions. With my first it was lamaze breathing and counting that helped me ride out each one. With my second the pain was different, constant back labor and the contractions felt a lot worse, so the bradley method of "stick to this position, don't move, and relax every muscle in your body" helped the most. So even between natural birth methods there's no perfect one. But it helps to be familiar with some, take a class, read a book, practice.

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).

public service announcement: eating lunch with your friend who's a new mom

Fri Sep 18 10:27 AM by Nachos in Pregnancy & Parenting

I honestly love holding other people's babies while they're eating. Did this for a new mum friend who needed to go to the loo and she turned back a few times because the baby was crying, but as soon as she was out of sight, baby was just fine.

let's discuss childhood injuries

Thu Sep 10 3:19 AM by DarkOne in Pregnancy & Parenting

When I was about 7 or 8 I was riding my bike and rode off the curb, flew off the seat, landed on the tube, and somehow managed to get a cut in my vaginal area. Apparently, it bled like a mofo. It turned out to be only a small cut, but I can only imagine what my mom felt like when her 8yo showed up at her door crying hysterically and bleeding profusely from her crotch.

OMG, twins! Advice & thoughts?

Sat Oct 3 8:19 AM by BethReed in Pregnancy & Parenting

Hi there,
I'm new here... and just starting my second trimester of pregnancy with twins. They are fraternal, conceived via IVF, and it's my first healthy pregnancy. We have had some painful fertility misadventures in the past, so I'm just grateful and excited to be this far along, with healthy scans and test results thus far!

As I start to realize that these twinsies are most likely here to stay (!), I'm also starting to feel pretty unprepared, especially in terms of what supplies we'll need. There seem to be a lot of irritating "essential baby supply" articles that are thinly disguised ads for things like $800 strollers, which are anyway designed for singletons, so they're not even relevant to our situation! And of course, I think my feeling of being unprepared on the logistical front is probably masking my feelings of unpreparedness on the emotional / physical / marriage / family front as well.

So... I'd love to hear... what do you think my hubby and I should know about parenting twins? Or parenting at all?

Altering my mind about pregnancy

Mon Sep 28 9:09 PM by ablotial in Pregnancy & Parenting

My mom was 39 when I was born and 42 when my younger brother was born both naturally with no drugs or complications, and both of us were healthy and happy. The only thing I will say is my mom was noticibly "less fun" (read: less energy) than my friends' moms, who were often in their early-to-mid 30s when my mom was 50.

As others have mentioned though, there is this (overblown) line in many minds about age 35, so they will probably want to monitor the crap out of you. And there is a bit of a higher risk for chromosomal problems, like Downs. But if you are ok with that and unlikely to have a super early menopause, I say you know yourself best and it is better to wait until you are at a point where you are capable and mentally ready to support a baby. 40s are definitely not too late!

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.

sleep deprivation

Thu Sep 17 3:26 PM by robichek in Pregnancy & Parenting

Apparently there is a new book out called The Rabbit Who Wants to Go to Sleep and it is ALL THE RAGE.. http://www.amazon.com/Rabbit-Wants-Asle ... 149617951X

If you are still breastfeeding, that may be an issue (not that I recommend that you stop breastfeeding). Topping up before they sleep could be warranted.

For my kid, we keep his room permanently dark and have a white-noise soundmaker outside his door and it works well. Also, although we didn't do a pure "cry it out" method, there were times at the beginning of his "sleep through the night" nights that he would wake up and whimper and we just would wait 2-10 minutes and he'd go back to sleep. That went away and now we don't expect a peep until at least 11 hours have passed. I think we as worried mothers are sort of complicit in creating their "need" for us.. If my boy knows I'll come in within 30 seconds of him making a sound, he'll want me to come in. If he's not sure how long it'll take, he'll go back to bed... Please take the preceding advice as something I might recommend only after the kid is 1 year old. Before then I would never just let him be... I used to rock him to sleep until my arms gave out.

non-religious parents raise okay kids after all

Wed Sep 2 3:37 AM by AndrewZealand in Pregnancy & Parenting

However, one thing I remember clearly from my childhood is being very afraid of death.

I remember when I first realized that we die when we get old. I told my best friend at the time and he got really upset. I never quite had the need for an afterlife, though I remember at about 10 I conceived that when we died maybe we came back again as someone else. This was before I knew that someone else had already thought of that :P

Presently I'm still open to the idea of some sort of afterlife when we die. One thing I find inspirational when I'm depressed is the thought that there might be some sort of new and interesting afterlife when we die. It can give me energy to keep on with this life. But the idea of eternal punishment for our sins in the afterlife? I'd say to the average man that's the same as "Life's a bitch and then you die and then you go to hell and suffer". The idea that we MIGHT get punished for our sins when we die, for most of us, only serves to make our lives more fearful and miserable and then of course when we die we may well find that no such punishment exists. Like the saying goes "Cheer up, it might never happen". The only usefulness I think of for the idea of afterlife retribution, is something with which to scare a tyrant who is so powerful that apparently nothing in this life can stop him or her abusing people and nature. By telling them that there might be something in the afterlife holding them to account, you might actually scare them into reconsidering their behaviour. But I'd say that for most people, hell or other afterlife punishment, is an unnecessary fear.