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
0601139W,5D
1167239W,5D
29238W,6D

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

Day 0 previous births 1 previous birth 2 or more previous births
#% #% #%
245 9 0.1 4 0.2 1 1.1
246 13 0.2 1 0.1 3 3.3
247 5 0.1 3 0.2 1 1.1
248 16 0.3 3 0.2 1 1.1
249 13 0.2 4 0.2 2 2.2
250 15 0.2 4 0.2 2 2.2
251 13 0.2 3 0.2 2 2.2
252 25 0.4 6 0.4 1 1.1
253 21 0.3 3 0.2 0 0
254 18 0.3 9 0.5 1 1.1
255 30 0.5 4 0.2 1 1.1
256 18 0.3 9 0.5 2 2.2
257 18 0.3 11 0.7 1 1.1
258 26 0.4 7 0.4 1 1.1
259 42 0.7 16 1 2 2.2
260 39 0.6 12 0.7 1 1.1
261 44 0.7 11 0.7 4 4.3
262 62 1 10 0.6 4 4.3
263 62 1 18 1.1 3 3.3
264 55 0.9 19 1.1 2 2.2
265 79 1.3 24 1.4 3 3.3
266 90 1.5 29 1.7 2 2.2
267 82 1.4 24 1.4 1 1.1
268 83 1.4 25 1.5 2 2.2
269 117 1.9 36 2.2 1 1.1
270 138 2.3 41 2.5 1 1.1
271 119 2 38 2.3 1 1.1
272 150 2.5 56 3.3 1 1.1
273 187 3.1 53 3.2 2 2.2
274 173 2.9 40 2.4 1 1.1
275 190 3.2 53 3.2 4 4.3
276 219 3.6 74 4.4 4 4.3
277 243 4 71 4.2 1 1.1
278 253 4.2 64 3.8 1 1.1
279 297 4.9 76 4.5 1 1.1
280 333 5.5 91 5.4 1 1.1
281 332 5.5 78 4.7 1 1.1
282 264 4.4 88 5.3 1 1.1
283 273 4.5 61 3.6 5 5.4
284 251 4.2 55 3.3 2 2.2
285 252 4.2 64 3.8 1 1.1
286 240 4 82 4.9 1 1.1
287 254 4.2 64 3.8 1 1.1
288 188 3.1 38 2.3 1 1.1
289 148 2.5 44 2.6 1 1.1
290 136 2.3 47 2.8 1 1.1
291 117 1.9 15 0.9 1 1.1
292 69 1.1 35 2.1 3 3.3
293 67 1.1 12 0.7 2 2.2
294 48 0.8 13 0.8 1 1.1
295 14 0.2 8 0.5 1 1.1
296 18 0.3 6 0.4 2 2.2
297 15 0.2 3 0.2 1 1.1
298 7 0.1 3 0.2 1 1.1
299 6 0.1 1 0.1 2 2.2
300 6 0.1 1 0.1 1 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

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
spacefem.com
spacefem@spacefem.com

Recent Comments

For more support, encouragement, Q&A,
you are invited to join our pregnancy forum. We are dedicated to answering complicated questions as logically as we possibly can.

Cry it out vs. Attachment

Thu Aug 18 9:39 PM by melsbells in Pregnancy & Parenting

Everything I remember reading about this seemed fairly contradictory. As far as I could tell, there aren't actually any good studies available, which I suppose is expected when the subject matter is infant behavior. We ended up leaning toward attachment parenting because it fit our personalities better and then the kid's personality seemed to pull us decidedly in that direction. I was even convinced by doctors and nurses in the hospital to bed share, even though I was opposed to it from what I now see as fear mongering in the U.S. about safety.


it gets better thread for parents

Fri Jul 29 1:26 AM by curlybae in Pregnancy & Parenting

It's true though. You don't get a good night's sleep until they're an adult. Looking back, I don't think my mom slept a wink my baby brother was born lol


Public Service Announcement: Furniture Anchors!

Fri Jul 1 5:17 AM by cwbyrvr in Pregnancy & Parenting

I do find it very interesting to find that IKEA is recalling furniture because people don't secure it to walls... I was given anchors with all of the stuff I've bought from there. Though maybe that's only a somewhat recent thing? It was over a half a year ago that I bought stuff from there, though.

And I didn't use the anchors... I guess I am about to be an aunt and it might be a good idea to get that stuff secured in case I ever end up watching my niece here. Hadn't really worried about it prior because there was literally never going to be a time I'd have kids running around this place. The likelihood of my niece ever being here is pretty damn low too, but I wouldn't say 0%.


Pregnancy Guidebooks for Cool Moms (humor)

Wed May 11 3:39 PM by rowan in Pregnancy & Parenting

One of my friends is part of this magazine. She didn't write this one but it's pretty hilarious.

http://theoffingmag.com/wits-end/pregna ... cool-moms/

Anyone want to come up with our own "Guidebook" titles?

spoiler:
the one about inducing labor by setting your hair on fire is my favorite


stuttering at the end of words

Thu Apr 28 9:21 PM by Sonic# in Pregnancy & Parenting

I don't think it's a cause for immediate concern, but my experience would lead me to talk to a doctor about it. I didn't have a stutter when I was 2. I had language delay. I didn't learn language when kids typically do. Needless to say, I was a very frustrated little kid. When my parents and my pediatrician caught on to it, I had to spend a few years seeing a speech pathologist.

I've read that stuttering is at least partly psychological, but I really can't say more than that. If nothing else, I'd just want to know that his frustration is a necessary part of development like teething and not something that can be avoided.


Do you use boil bags?

Mon Aug 8 3:20 PM by Sonic# in Pregnancy & Parenting

All I can think of is the Arrested Development episode where George Bluth Sr. has a hot tub installed in his attic hiding place so that he can boil food in the bag.

I've not boiledfood in a bag, but not due to any aversion.


How NOT to help parents with a newborn

Fri Jul 22 6:11 PM by missmay in Pregnancy & Parenting

oh, lord, i am so tired of people telling me to "enjoy it!" regarding my newborn. and asking me if she is a "good baby." how could a baby be bad? and how does it make me enjoy her more to be reminded to do so? i imagine that if i was depressed and had a colicky baby these questions would be even more obnoxious.


Kids are full of LIES

Sun May 22 2:29 PM by rowan in Pregnancy & Parenting

Don't be silly monk, everyone knows a good conglomerate goes better with milk


young kids, consent and bodily autonomy

Mon May 2 8:46 PM by monk in Pregnancy & Parenting

I think it depends on the child. Do you ask a newborn if it needs it's diaper changed? Obviously no. Do you help a 13 year old get dressed? Obviously no. Somewhere in there the child matures to the point where they should learn that they own their bodies even if they don't have all the privileges an adult does. This age is going to vary wildly between different children (by widly, I would say 3 year range). I would guess that some kids might be ready for this lesson as early as 3 while others not until they're about to go school. Actually that's a pretty good benchmark, because if you're going to put them in a group with other kids they don't know they should know the rules about touching or being touched.

As far as kissing Grandma, maybe they shouldn't have to do it, but making them do this social communcation helps get them ready for all the other social communication that will be forced on them later. Plus, grandma needs that love to help HER be healthy.


test out my new name picker

Fri Apr 15 11:41 PM by rowan in Pregnancy & Parenting

my neighbor's dog is named Gunner? Makes more sense for a bird dog....


How to Announce Baby #4?

Mon Aug 1 10:44 PM by rowan in Pregnancy & Parenting

eh go ahead and eat them. I recommend ketchup, though. Or hot sauce.

Oh and congratulations and all that, it'll be fine. :) People do get weird but here are some random responses:
"well, I guess we were wrong!" cheerfully
"Shit, we we were wrong. OH well.
"IDK do you think we should get rid of one of the others?"
"You want one?"
"Well we weren't sure the first three would be enough sustenance when the apocalypse comes"
"We have W girls and Z boys, we're hoping for something in between"
"Actually we're hoping it's a cat"
"Did you realize that we just sent a new spacecraft to Jupiter? Isn't that exciting!"


Denial Trends?

Thu Jul 21 7:24 PM by SameMom in Pregnancy & Parenting

Sorry, if I was not able to mention other details. Actually, I have a (US) medicare insurance. And I am particularly referring to this denial management software which help clients with their claims. Thanks for your reply. :)


minimalist kids birthday parties

Sun May 15 9:19 PM by melsbells in Pregnancy & Parenting

For the first two birthdays, we tried to make some special food and splurge on interesting fruit. If I lived closer to friends, I would want to throw a get-together as an excuse to see my friends, celebrate our survival of another year. I don't think my kid would actually be into that though. They are turning out to be quite introverted and shy.


7th Birthday - Theme Suggestion

Mon May 2 8:18 PM by melsbells in Pregnancy & Parenting

I met a kid about that age who had a geo-caching birthday party and another one who had a roller-skating party. Both sounded awesome. The first one sounded like a whole lot of work to put together.


Criminalization of drug use during pregnancy to end in Tennessee

Tue Mar 29 2:48 AM by Amuro in Pregnancy & Parenting

Actually I think it would be better if the penalty was rehab and being sent to a therapist. It's pretty dumb to criminalize it especially because there is an almost certainty custody will be taken away. Why not try and fix the problem with addiction so there is less of a strain on social services. There should be increased awareness but what idiot doesn't know drugs are bad for you and worse if your pregnant. If you are addicted to drugs you obviously are thinking about getting high and that's about it.