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 0.1 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 0.3 4 0.2 2 2.2
251 13 0.2 3 0.2 2 2.2
252 23 0.4 6 0.4 1 1.1
253 20 0.3 3 0.2 0 0
254 17 0.3 9 0.5 1 1.1
255 29 0.5 4 0.2 1 1.1
256 18 0.3 9 0.5 1 1.1
257 17 0.3 11 0.7 1 1.1
258 26 0.4 6 0.4 2 2.2
259 41 0.7 16 1 2 2.2
260 38 0.6 12 0.7 1 1.1
261 43 0.7 11 0.7 4 4.4
262 62 1 10 0.6 4 4.4
263 62 1 18 1.1 3 3.3
264 52 0.9 19 1.2 2 2.2
265 79 1.3 24 1.5 3 3.3
266 90 1.5 28 1.7 2 2.2
267 81 1.4 24 1.5 1 1.1
268 82 1.4 24 1.5 2 2.2
269 115 1.9 35 2.1 1 1.1
270 136 2.3 39 2.4 1 1.1
271 119 2 38 2.3 1 1.1
272 148 2.5 56 3.4 1 1.1
273 183 3.1 53 3.2 2 2.2
274 171 2.9 40 2.4 1 1.1
275 187 3.2 52 3.1 4 4.4
276 217 3.7 72 4.4 4 4.4
277 242 4.1 71 4.3 1 1.1
278 248 4.2 64 3.9 1 1.1
279 294 5 75 4.5 1 1.1
280 325 5.5 89 5.4 1 1.1
281 327 5.5 76 4.6 1 1.1
282 256 4.3 85 5.1 1 1.1
283 273 4.6 61 3.7 5 5.5
284 249 4.2 54 3.3 2 2.2
285 251 4.2 64 3.9 1 1.1
286 235 4 81 4.9 1 1.1
287 250 4.2 64 3.9 1 1.1
288 187 3.2 38 2.3 1 1.1
289 147 2.5 44 2.7 1 1.1
290 135 2.3 47 2.8 1 1.1
291 116 2 14 0.8 1 1.1
292 69 1.2 34 2.1 3 3.3
293 67 1.1 12 0.7 1 1.1
294 47 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 0.3 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
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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.

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.

it gets better thread for parents

Mon Jun 13 11:16 PM by rowan in Pregnancy & Parenting

Sometimes I think they go through a growth spurt or something weird-ass like that and then when it's done it gets better again. But who knows, kids are weird. It will get better again. We seem to be having night terrors again though not as badly as before. Those seem to correlate with growth spurts (for us). They are tailing off again so hopefully...?

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?

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

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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.

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.

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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. :)

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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

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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.

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When my son's cord fell off a few weeks ago, I put it on the counter next to his changing pad then promptly forgot about it. I've no idea where it is now.

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Tue Mar 22 2:34 AM by codingchemist in Pregnancy & Parenting

It's really obvious that the dog is stressed out by the toddler and I'm worried about the times when the kid is showing too much interest in the dog and we're not paying enough attention.

Yeah-stressed out dogs can make for stressful times with toddlers. Toddlers just want to pet and hug, and don't want to listen to pleas to leave the dog alone. We wound up getting an automatic treat dispenser and giving the dog relaxation exercises behind a barrier. So now when the toddler is running the length of the house screaming, the dog can be behind the barrier staring lovingly at her treat dispenser, instead of trying to herd the kid. Of course, it isn't really feasible to keep the dog behind the barriers at all times, but at least it gives us some breaks.

Our behavioralist also recommended Prozac, which took a long time to get the dose right, but has helped the dog be less reactionary and neurotic.

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

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Thu Mar 10 6:56 PM by rowan in Pregnancy & Parenting

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