Length of pregnancy for first time vs. second & third time moms

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Cumulative Distribution

This graph shows the percentage of births that'd occured on or before a given day, like a running total.

There's a popular rumor out there that first time moms are very likely to go past their due dates... sometimes even way past. I had people tell me to expect to go a week overdue. Here's what I found in the survey. Considering only births resulting from spontaneous labor:

So why the rumor about first time moms going late? There are articles all over the web that say the average first time mother goes eight days overdue and doesn't have her baby until 41 weeks, 1 day. I say hold off, people. There's only one study that suggested this median: The length of uncomplicated human gestation (Mittendorf et al). The study was simply a record of a few hundred women at one private practice. Only 31 of those women were first time mothers. The study itself calls for further research. You can read it here. Given the small sample size and drastic deviation from the large-scale studies cited in my introduction, I'd suggest we dispell the rumor a bit and calm everybody down. And teach bloggers to read medical studies. Due date statistics are much better looked at as an epidemiology topic than a medical one, anyway.

Average length of pregnancy, by number of subsequent previous births

# of previous births# of birthsAverage Arrival

Daily probabilities of spontaneous labor, by number of subsequent previous births

Day 0 previous births 1 previous birth 2 or more previous births
#% #% #%
245 8 0.2 3 0.2 1 1
246 12 0.2 0 0 3 3.1
247 3 0.1 3 0.2 1 1
248 14 0.3 3 0.2 1 1
249 12 0.2 3 0.2 2 2
250 14 0.3 3 0.2 1 1
251 11 0.2 3 0.2 2 2
252 20 0.4 6 0.4 0 0
253 17 0.3 2 0.1 0 0
254 14 0.3 9 0.6 1 1
255 29 0.6 4 0.3 1 1
256 16 0.3 9 0.6 1 1
257 14 0.3 11 0.8 1 1
258 23 0.5 4 0.3 2 2
259 34 0.7 13 0.9 2 2
260 35 0.7 6 0.4 1 1
261 30 0.6 9 0.6 4 4.1
262 58 1.2 9 0.6 3 3.1
263 56 1.1 17 1.2 3 3.1
264 43 0.9 17 1.2 2 2
265 69 1.4 20 1.4 2 2
266 71 1.4 24 1.7 2 2
267 72 1.5 19 1.4 1 1
268 67 1.3 19 1.4 2 2
269 99 2 29 2.1 5 5.1
270 121 2.4 31 2.2 1 1
271 105 2.1 31 2.2 1 1
272 127 2.6 50 3.6 1 1
273 152 3.1 44 3.2 2 2
274 148 3 32 2.3 1 1
275 152 3.1 44 3.2 4 4.1
276 174 3.5 61 4.4 4 4.1
277 203 4.1 60 4.3 1 1
278 213 4.3 54 3.9 1 1
279 241 4.9 62 4.5 1 1
280 251 5.1 70 5.1 1 1
281 262 5.3 60 4.3 1 1
282 211 4.2 76 5.5 1 1
283 226 4.6 51 3.7 5 5.1
284 213 4.3 45 3.2 2 2
285 211 4.2 54 3.9 1 1
286 202 4.1 68 4.9 4 4.1
287 203 4.1 58 4.2 1 1
288 155 3.1 32 2.3 1 1
289 120 2.4 39 2.8 1 1
290 114 2.3 40 2.9 1 1
291 104 2.1 14 1 1 1
292 58 1.2 26 1.9 2 2
293 56 1.1 10 0.7 1 1
294 41 0.8 11 0.8 7 7.1
295 13 0.3 7 0.5 1 1
296 17 0.3 5 0.4 2 2
297 9 0.2 1 0.1 1 1
298 6 0.1 3 0.2 1 1
299 5 0.1 0 0 1 1
300 3 0.1 0 0 1 1

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

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EDD Icon Generator
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Recent Comments

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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.

Happy Birthday, Baby Bird!

Thu Aug 27 12:57 PM by ablotial in Pregnancy & Parenting

Cute kids, and I love the cupcake theme!

mom guilt bingo

Sun Aug 23 11:06 AM by ohbonobogirl in Pregnancy & Parenting

Adding to the newborn edition:
Knowingly fell asleep on the couch with baby on your chest because...sleep...
Used a tablet or phone to surf net or play games while nursing rather than cherishing every moment
Took baby out in public solely because it's really great to hear strangers compliment your adorable little being.

teething necklaces

Tue Aug 4 7:12 PM by DarkOne in Pregnancy & Parenting

They'll chew on anything and everything. And I think you may have the scale wrong. These are large silicone beads. Think of it as a teething/chewing toy that is long enough to be worn around mom's neck. This isn't a string of pearls that will break if you tug on it and you'll and up with a mess of pearls on the floor or in baby's mouth. Google images for "babies and chew beads".
And these are not at all related to those dumb amber necklaces...

do you have a parenting goal?

Sun Jul 26 5:16 PM by rosefox in Pregnancy & Parenting

I'm definitely in the "happy healthy child is my only goal" camp. I know some people who are superstars in their chosen fields. From what I can tell, being a superstar is completely exhausting and often really psychologically taxing. So if my kid isn't naturally inclined that way, I certainly don't intend to push them.

If I'm hoping for anything, I hope they'll end up being a quirky artsy/techy type like just about everyone else in the family. But my brother's a corporate lawyer and I love him anyway, so even if our kid turns out to be a white sheep, they'll just bond with Uncle A and it'll all be fine. :)

Braxton Hicks questions

Fri Aug 28 5:10 PM by geerlady in Pregnancy & Parenting

Was just in the hospital for a pre-term labour false alarm earlier this week.

My Braxton Hicks have changed this time around to be sometimes painful, period-like cramps extending to my lower back with contractions that are fairly regular (every 10 to 15 minutes regardless of what I am doing). But they have been medically confirmed to still just be Braxton Hicks :o

As a general comment, my BH have always been symmetric, so symmetry was never a good marker for me.

So... second time around and I still can't tell the difference :)

What new thing did your baby do this week?

Mon Aug 24 5:56 PM by Nachos in Pregnancy & Parenting

It's not my baby, but my sister taught my best friend's baby how to fist bump and I taught her how to say yolo. Heh.

What the Hell just entered my earhole?

Mon Aug 17 11:34 AM by DarkOne in Pregnancy & Parenting

Oh that reminded me of a friend's kid... My friend, her 6-yr old son and her 9-yr old daughter were driving to school in the morning, when the son proudly declared "When I grow up I want to be a racist." My friend said she went O_O and her daughter went on a rant as to how horrible of an idea that was... The son was all sad and confused when he responded "But I really love race cars!"

That cracked me up.

Kids books we hate

Mon Aug 3 1:10 PM by Sonic# in Pregnancy & Parenting

I absolutely loved at least one of the Berenstain Bears books (one based off of the Tortoise and the Hare), but I also hate at least one of them. The Berenstain Bears and the Trouble With Chores features a Mama Bear exasperated that no one is doing chores, with the kid bears and Papa Bear keep excusing themselves out of doing them. Rather than coming to some equitable distribution of labor by the end, the house gets really bad and the kids and pop finally binge clean. The situation didn't seem solved; Mama Bear just seemed pacified.

for all you moms and future moms

Tue Jul 21 2:28 AM by DarkOne in Pregnancy & Parenting

Eek and here I was about to say this felt like a tame version of Mother Lover.

Body Betrayal

Thu Aug 27 8:44 PM by mathnome in Pregnancy & Parenting


I'm currently 37 weeks, and while almost all my immediate coworkers have been super supportive of my being at work (and planning to continue being at work), I got a "tsk tsk" from another woman the other day about not taking my leave yet, and she hasn't been the first one. (Interestingly, so far it's only been the women who seem to think I'm supposed to leave already.)

Anyway, my line has been, "I'm really enjoying my project right now. I might as well enjoy it while I can!" (Which is true, and also generally shuts them up.)

Good luck! I hope the BP stays under the magic threshold.

What did you hate hearing when you were pregnant?

Mon Aug 24 11:52 AM by DarkOne in Pregnancy & Parenting

3. I think you're putting on too much weight. You know it''s going to make it harder to get your body back. (I'm 6 months post baby today, so now the comments have turned to "I told you!")


let's list our favorite children's books

Tue Aug 11 3:49 AM by DarkOne in Pregnancy & Parenting

As a grade school kid, I spent a lot of time perusing dictionaries, atlases and world almanacs. I blame it on my Big Bird's Sesame Street Dictionary, which was awesome:


Did anyone else own these? I have the full set stashed somewhere for when my kid is old enough.

RIE parenting

Sat Aug 1 2:09 AM by ohbonobogirl in Pregnancy & Parenting

I encountered this randomly the other day. Now I need to go dig some more into it...

the wonder weeks: practical help for parents or self-fulfilling quackery?

Mon Jul 20 2:29 AM by geerlady in Pregnancy & Parenting

I am actually a big fan of the wonder weeks. I found it better than most popular level books looking at infant development. My son was right on the clock for the fussy/happy times, and it was interesting to look for the new skills that I might not have noticed if I hadn't been watching for them.

On the other hand, I stumbled upon this accidentally, so didn't have any hype surrounding it. I also read the author biographies before picking up the book, so took everything with a grain of salt knowing that 1) the authors specialized in chimpanzees, not humans, and developed their theories based on chimps; 2) the bulk of the research is greater than 20 years old and so much of the book is out-dated; and 3) given #'s 1&2, incidental parenting advice had to be filtered.

That said, I still found the book useful and regularly recommend it to people.