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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 8654 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 8654 records:

Twin births make things slightly out of the norm, so they are not included in the chart. This excludes 82 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 267 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.

Tips and pitfalls for gender-neutral parenting

Sun Mar 29 1:53 PM by melsbells in Pregnancy & Parenting

One of the biggest problems I'm noticing is my own use of gendered pronouns. I use a gendered pronoun all the time when I could easily do otherwise. I appreciate having a community here where I can talk about my kid and practice not using gendered pronouns.

My spouse and I aren't quite on the same page with this. I think we should be doing more to subvert social expectations, like have a doll, but my spouse sees that as just one more THING we don't need (we're fairly anti-consummerist).

The kid is less than a year old, so there's lots of time to see if the things we do make a difference. We have clothing intended for boys, clothing intended for girls, as well as gender neutral clothing.


Interacting with parents whose choices you don't trust

Thu Mar 26 3:08 PM by rowan in Pregnancy & Parenting

How can you tell who has been vaccinated or not?
That's the tricky part, you can't really. So unless the parents say something you never actually know.


How did you decide how many kids to have?

Fri Feb 27 2:42 AM by MFS in Pregnancy & Parenting

First 2 were on purpose, 3rd kinda not so much, 4th was totally an oops (when you think things are broken, they have this habit of fixing themselves). Made sure we were done after that. Aside from that, we knew we'd have at least 2, likely 3, but the timing was not at all what we thought we were "planning". Such is life.


name trends

Wed Jan 28 8:58 AM by Lemon in Pregnancy & Parenting

Some of my family members who work in the healthcare system have overheard some doozies.

I work in healthcare, so I can relate. I think my favourite was a kid called "Abcde" which we were assured was pronounced "Abasidee". Mirena was a cute choice, named after the contraceptive device. Then there have been some good narrative names including places of birth - e.g. "Shell" (after the baby born in a petrol station carpark) - and places of conception - e.g. "...(a suburb which I'll keep secret - just in case this somehow breaches some poor kid's privacy) bus stop."


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Thu Jan 15 9:17 PM by bigdaddy in Pregnancy & Parenting

I will do cheers but in people's experiences and options I should be ok


Advice for non-parents?

Fri Mar 27 9:04 PM by rowan in Pregnancy & Parenting

only offer to bring food if that's truly your thing and you truly can cook.
oh yeah. heh. I live in a bubble of good cooks I guess. ;)


How Good Parents Miss Child Sexual Abuse and 5 Questions to Change That

Tue Mar 24 3:20 AM by rowan in Pregnancy & Parenting

And that includes parents too - I try to always ask "can I have a hug/kiss" rather than "give me a hug/kiss" and that she can always always say no if she doesn't want to, even with me. (She's old enough now that I've had enough practice that even writing "give me a hug" sounds weird to me)

Also I think on a more general level it's good to practice asking these kinds of questions anyway. Not just for abuse, but in general if you want to know what's going on in their life. Asking "how was school" gets an "I dunno" even now, but asking more specific things like "What was something funny that happened today" or "What did you do at recess?" really gets her talking more. Not sure where I read that, but the kinds of questions above made me think that sometimes just asking a better question will help get at what you want to know more than asking generalities.

The other thing is definitely definitely not making judgment on those things that they're saying. Instead try "What did you think about that ?" or "What did you do when happened?" "What would make you feel safe?" "What could you try if that happens again?" "Is there anything you'd like me to do?" These are things that I've asked wrt other kids on the playground (pushing etc) but I think it's the same idea in terms of empowering the child to talk about things that happen and letting them help come to the conclusions. Which isn't to say you can't help them get there by suggesting actions they could do, etc, just that if you get them talking about things that happen on a daily basis then if something weird happens then they'll likely be more comfortable telling you, especially if you've already shown them what boundaries there are and respecting those boundaries (like not tickling if they say they don't want to be tickled etc).


Bonding

Fri Feb 6 4:35 AM by Zeph in Pregnancy & Parenting

Bonding with my future infant is a concern of mine, because my depression is debilitating. I'm relieved to know that normally functioning women don't feel that head-over-heels mushy baby love.


34 Weeks

Fri Jan 23 11:57 PM by Sonic# in Pregnancy & Parenting

Probably not. As we pointed out in the other topic, 34 weeks isn't even on the chart because it's so rare:
http://spacefem.com/pregnant/charts/duedate1.php

You'd be better off worrying over a car accident.


Why Pocket Diapers Are Better Than Disposables

Sun Jan 11 3:31 PM by Nancy Erick in Pregnancy & Parenting

Hi everyone

A Beautiful article by writer Kathryn McDowell recently I read liked to share with you

Why Pocket Diapers Are Better Than Disposables :


As a young parent, you've no doubt heard of pocket diapers. Perhaps a neighbor or a co-worker told you about them. Or it was your babysitter, who asked you to get them soon. Well, so many families in America are today turning to pocket diapers for their little ones. But a majority still holds on to disposable diapers, while being curious about pocket diapers at the same time.

Why are diapers so great? For a start, pocket diapers come with brand new fabrics and super absorbent pads, are easy to put on, simple to take off, easy to wash and quick to dry. And they are better than disposable diapers.

They are Cheaper
Until your child is potty trained, you'd probably spend something close to $2500 to $3000 on diapers - which is quite a lot. Biodegradable disposable diapers are even costlier. But with pocket ones, you won't have to spend anything more than $350 to $400, till your child is potty trained.


Bilingual Babies

Fri Mar 27 2:18 AM by Nae in Pregnancy & Parenting

As someone who only knows English, I am so impressed by people who are bilingual, trilingual, or multilingual. Only knowing English feels like you can only understand part of the world. The problem I've had is I don't really know what language I should try to learn.

I think it's a wonderful thing to be able to teach a child two languages. The part others have raised about being able to communicate with family members makes it that much more rewarding.

Don't worry about the naysayers - they always have to pick on something, and if it wasn't that, they'd just find something else. ;)


It takes 13 million calories to raise a child!

Sun Mar 15 6:33 PM by Metcodon1 in Pregnancy & Parenting

Child care is BEYOND insanely expensive... When my husband and I were deciding to to have children we agreed to both go part time (4 days a week) in order to make our lives easier (we both make more than we would pay for day care, so we did take a financial hit with this decision). For example, whoever is home on a given day can throw in some laundry, or some food into the slow cooker, run errands... All of these things multiply in number once you have kids at the same time becoming more difficult to schedule. Unfortunately, when my daughter was around 1 my work situation changed and I had to go full time... let me tell you, it was definitely easier when I wasn't.

Anyway, the point I am getting to here is, when kids are little they require a lot of attention and sometimes (often?) some career sacrifice but it's TEMPORARY. When we decided to get pregnant we agreed that we would both slow down career wise for 5 years. For us it means, we do our jobs, we come home. We don't sign up for extra projects, we try to limit our travel, but it's all just for now. We also decided to have our kids close together for that reason (they'll be 2 years apart). Anyway, it's something to keep in mind. An 8 year old goes to school and doesn't need constant monitoring to keep from killing him/herself. It's ok to decide to slow down for a period of time, knowing that you'll come roaring back once they're older.


Because judging other parents is absurd.

Tue Feb 3 7:05 AM by Zeph in Pregnancy & Parenting

It's alright to discuss these topics, but there needs to be boundaries. I would use the following, personally:

1. Don't insert yourself into a stranger's situation or bring up the topic yourself to a stranger.
2. It's probably not your business if you're not a parent yourself.
3. Ask the person why they are making these choices, don't assume.
4. Try to end on a positive note.

I don't feel right bringing up to my sister that I don't agree that she spanks my nephew, but after being a teacher, I am pretty forgiving of parents' choices. It's the hardest role there is. Even though I want to raise my children on nutritious meals, cloth diapers, and museum passes, I know I'm going to slip up and give them fries, Huggies, and Disney DVD's.

I also feel that home birth is a fine option for low-risk births, where the mother is well coached and the midwife is reputable. If you live near a hospital and your midwife is highly qualified, why not? They have equipment. I won't be choosing this option, but with any birth plan there needs to be a series of precautions taken.


sleep humor

Fri Jan 23 7:13 PM by rowan in Pregnancy & Parenting

Here's a really funny humor bit about sleep, but the same general idea can apply to pretty much any "advice" given to new parents.

"I bought all the top books on baby sleep and development. I read through them all, as well as several blogs and sleep websites. I gathered lots of advice:"

https://chimericalcapuchin.wordpress.co ... babybooks/


some quotes recently I read

Sat Jan 10 4:50 PM by Nancy Erick in Pregnancy & Parenting

i search about these quotes in google