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

[ Survey Results Intro | All Charts | Take the survey ]

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
0544139W,5D
1152039W,5D
29739W,1D

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 0.3 1 1
246 12 0.2 0 0 3 3.1
247 3 0.1 3 0.2 1 1
248 15 0.3 3 0.2 1 1
249 13 0.2 3 0.2 2 2.1
250 14 0.3 3 0.2 1 1
251 13 0.2 3 0.2 2 2.1
252 22 0.4 6 0.4 0 0
253 19 0.3 3 0.2 0 0
254 17 0.3 9 0.6 1 1
255 29 0.5 4 0.3 1 1
256 17 0.3 9 0.6 1 1
257 15 0.3 11 0.7 1 1
258 25 0.5 5 0.3 2 2.1
259 38 0.7 14 0.9 2 2.1
260 37 0.7 12 0.8 1 1
261 36 0.7 9 0.6 4 4.1
262 60 1.1 9 0.6 4 4.1
263 59 1.1 18 1.2 3 3.1
264 48 0.9 18 1.2 2 2.1
265 72 1.3 23 1.5 3 3.1
266 80 1.5 28 1.8 2 2.1
267 76 1.4 20 1.3 1 1
268 77 1.4 24 1.6 2 2.1
269 107 2 34 2.2 1 1
270 129 2.4 34 2.2 1 1
271 109 2 32 2.1 1 1
272 138 2.5 52 3.4 1 1
273 168 3.1 49 3.2 2 2.1
274 155 2.8 36 2.4 1 1
275 168 3.1 48 3.2 4 4.1
276 195 3.6 64 4.2 4 4.1
277 221 4.1 61 4 1 1
278 232 4.3 58 3.8 1 1
279 267 4.9 70 4.6 1 1
280 285 5.2 80 5.3 1 1
281 291 5.3 70 4.6 1 1
282 232 4.3 81 5.3 1 1
283 248 4.6 54 3.6 5 5.2
284 235 4.3 51 3.4 2 2.1
285 231 4.2 59 3.9 1 1
286 222 4.1 76 5 4 4.1
287 227 4.2 59 3.9 1 1
288 169 3.1 36 2.4 1 1
289 141 2.6 42 2.8 1 1
290 124 2.3 47 3.1 1 1
291 110 2 14 0.9 1 1
292 64 1.2 31 2 2 2.1
293 62 1.1 11 0.7 1 1
294 43 0.8 13 0.9 8 8.2
295 13 0.2 7 0.5 1 1
296 17 0.3 5 0.3 2 2.1
297 11 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

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.

spacefem's guide to baby-led weaning

Thu Feb 11 11:42 AM by volvogrrl in Pregnancy & Parenting

Hi, new here from Sydney, I am here to share that my baby is touchy about blanket. Whenever I changed her blanket she never sleep and get annoyed. I don't know whats this. But after reading your post that your kid eat everything expect Indian, I think kids have sense of liking and disliking in this early age!


the unplanned homebirth

Tue Feb 2 11:37 AM by Aum in Pregnancy & Parenting

My great aunt had a baby while on the toilet... but then, she had 19 children in total so I'm sure by baby 10 it wasn't that big of a deal.


High needs newborn support thread

Fri Jan 8 5:12 PM by rowan in Pregnancy & Parenting

One thing I do now that I wish I had done earlier on (though not sure I could have done in the first 6 months due to brain fog), is if I find myself thinking "I wish I could do X" or "I wish I had time for..." even if it's a fleeting thought, I write it down. Well actually I put it in my toodledo list, which I can access on my phone/computer/wherever. Then when I find time I'm like soooo what do I do again? Oh yeah! I wanted to do this thing! I mean, as simple as "read book" or "bake a thing" or something.

Early on though it might be good to look at the list though. Just add things to it. Eventually it gets better and time will come back.


How many sheets/clothes/blankets etc for newborn?

Tue Dec 29 4:41 PM by rowan in Pregnancy & Parenting

^This reminded me, we got two receiving blankets that were made for us, and they were bigger than the little ones that people buy. THOSE were way more useful for much longer.


Vent, advice: How to not be crazy at work...or in general

Wed Nov 4 3:23 PM by Maro in Pregnancy & Parenting

So, my pregnancy hormones are causing me major mood swings right now. I don't necessarily feel ashamed of it, but I'm frustrated enormously by it because I've got a ton of projects at work and a two-year-old to deal with, and I honestly do not have time to sit and cry for 30 minutes every few hours. I really hate being pregnant, for that and other reasons, but right now it's mostly that.

This morning I felt awesome and totally zen and amazing and only vaguely nauseated, and then a couple of things hit at work that I routinely eat for goddamn BREAKFAST and I'm a puddle of tears in my office. Luckily, I work from home so no one can see me, and all I can really do is push through it and keep getting stuff done. But it majorly sucked. Maybe there's no way around it...but if anyone has any ideas for coping with this stuff I'd be grateful. When you Google pregnancy hormones and mood swings the majority of advice is aimed at husbands "surviving" it, which immediately sends me into a rage spiral.


The pregnant woman's business trip from hell (or at least heck)

Sat Feb 6 5:09 PM by lyra211 in Pregnancy & Parenting

Yeah, I went back and checked some sources -- here's the table that I sort of remembered from when I was going through the ectopic scare:
http://www.advancedfertility.com/ectopic.htm
So, half of ectopic pregnancies never get above an hCG level of 1,000, and only 9% ever make it above 10,000. Needless to say, the ones that do keep rising to those very high levels tend to be the scary ones, since those are the ones that keep growing and can cause things like tubal ruptures.

I can't link to it because it's subscriber only, but Up To Date (which has reviews of the most recent medical studies) has the following to say about the hCG threshold of 2,000 (some practices use 1,500 instead) and diagnosis of ectopic pregnancy:

The diagnostic criteria depend upon the relationship to the hCG discriminatory zone (serum hCG level above which a gestational sac should be visualized by TVUS if an IUP is present). The hCG level of the discriminatory zone varies, but in most institutions it is 1500 to 2000 IU/L (see 'Discriminatory zone' above and 'Clinical protocol' above):


not excited about others' pregnancies

Fri Jan 22 8:53 PM by lyra211 in Pregnancy & Parenting

This thread makes me want to make a graph of my excitement levels about other people's pregnancies. It would start off quite mild through teenage years and college, start to spike in early adulthood when I started thinking about maybe being pregnant someday but it was still a distant future possibility, reach a high right around the time my first friends started getting pregnant, drop a little when my husband and I started trying and encountered issues (took us 11 months first time around with my irregular cycles), reach a frenzied crescendo around the time I got pregnant (I was super-excited about being pregnant at the same time as one of my best friends -- we were due with baby girls within a week of each other). Then it dropped like a rock to the depths of deepest despair when our first daughter died at 18w2d in my first pregnancy -- I just couldn't bear to see other pregnant women, especially smug pregnant women who were rubbing their bellies all the time and had the nursery painted by the middle of the second trimester. Now that I'm 36 weeks pregnant with our second, a son, my excitement levels about other people's pregnancies are generally about where they were in teenage/college years, although now tinged with a huge amount of cynicism. Since everyone I know is pregnant at my age, I couldn't care less if someone I know is due within a few weeks of me -- I've been there before. Someone pregnant for the first time and over the moon? I remember feeling that way. It didn't end well. I'm >8 months pregnant, and I still get intensely jealous of first-time pregnant women who have never experienced problems. Heck, I still feel stabs of jealousy when I see other pregnant women rubbing their bellies. I've spent my whole pregnancy mentally keeping my kid at arms' length, not really believing that he's going to be born. I'm still emotional and excited about my pregnancy, but it's all dulled considerably compared to what I felt the first time around -- and I spend much more time worrying and educating myself about potential problems.


Toys that make noises

Wed Jan 6 8:44 PM by melsbells in Pregnancy & Parenting

I definitely don't like these types of toys. I was so grateful when my brother asked how we felt about electronic toys before purchasing one for our kid. Unfortunately, most people don't ask, and even if they know don't care. Our general policy is to not replace batteries when they die, but we haven't caused any to die myteriously. The only batteries we have in the house are attached to a phone, a laptop, and a couple spares for a flashlight.

Just today I heard about this small study on the effects of electronic toys, which didn't really surprise me.

Results Among the 26 parent-infant dyads, toy type was associated with all outcome measures. During play with electronic toys, there were fewer adult words (mean, 39.62; 95% CI, 33.36-45.65), fewer conversational turns (mean, 1.64; 95% CI, 1.12-2.19), fewer parental responses (mean, 1.31; 95% CI, 0.87-1.77), and fewer productions of content-specific words (mean, 1.89; 95% CI, 1.49-2.35) than during play with traditional toys or books.


holiday shopping lists - kids/babies edition!

Mon Dec 14 8:49 PM by ladyhawk in Pregnancy & Parenting

We had something similar happen on the oldest's birthday last year. We had a talk with him about how he was fortunate, and had a lot of things, and asked him to consider sharing with less fortunate kids. He ended up leaving half of them unopened and we took them to the children's hospital.


Childbirth/infant cpr/baby care classes

Mon Nov 2 11:25 PM by antfancier in Pregnancy & Parenting

Thanks everyone! Lots for me to think about...

I have just started prenatal yoga and it seems that it will be pretty useful - although it's way more physical hard work than I expected (I'd better get used to it now, I guess).

Also, I hope I get an inflatable baby from whichever course I end up going with.


Group B Strep

Wed Feb 3 2:02 AM by lyra211 in Pregnancy & Parenting

It's not at my (US) hospital, alas, and I got the impression it is not common. :-( I am just planning to eat a bunch of yogurt and other probiotic stuff after birth and hope for the best (there's apparently some evidence that good bacteria can be passed through breast milk, so hopefully as I rebuild my own microbiome I'll also be helping my baby to build his). I'm simply not willing to take the risks associate with passing up antibiotics, so I think focusing on rebuilding a healthy microbiome is probably the best I can do.


'Forest' preschools

Mon Jan 11 3:21 AM by rowan in Pregnancy & Parenting

It's currently -22 C here and didn't get above -18. The wind was pretty bad too, which makes it worse. *shrug* Brisk activity would have been fine, like sledding or skiing, if it hadn't been for the wind, but trying to actually learn things I think would be much more challenging. Not the least of which is it would damage any equipment you were trying to use. I don't think you could really sustainably do it for the entire winter, or at least not for January and February.


speaker tampon lets you play music to your fetus

Tue Jan 5 5:36 PM by Aum in Pregnancy & Parenting

^ You'd have to pick something like heavy metal or drum & bass. Your kid would turn out all punk as a result, so it's win-win!


disapproving babies

Mon Dec 7 3:04 AM by Bork in Pregnancy & Parenting

I think your baby totally wins the "disapproving baby" contest. Her face is amazing. She's got the eyebrows and the mouth and oh man.


Postpartum girdle?

Fri Oct 23 2:47 PM by rowan in Pregnancy & Parenting

Interesting to hear everyone's stories about recovery. I still have issues with some of my ab muscles, I wonder if it would have helped.