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
0907339W,5D
1241539W,5D
29639W,0D

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

Day 0 previous births 1 previous birth 2 or more previous births
#% #% #%
245 14 0.2 6 0.2 1 1
246 17 0.2 1 0 3 3.1
247 12 0.1 7 0.3 1 1
248 18 0.2 3 0.1 1 1
249 18 0.2 4 0.2 2 2.1
250 17 0.2 6 0.2 1 1
251 19 0.2 3 0.1 3 3.1
252 34 0.4 9 0.4 1 1
253 26 0.3 4 0.2 1 1
254 24 0.3 12 0.5 1 1
255 35 0.4 8 0.3 1 1
256 23 0.3 11 0.5 2 2.1
257 30 0.3 15 0.6 1 1
258 42 0.5 12 0.5 1 1
259 62 0.7 21 0.9 2 2.1
260 57 0.6 16 0.7 1 1
261 58 0.6 18 0.7 4 4.2
262 77 0.8 15 0.6 1 1
263 87 1 24 1 3 3.1
264 75 0.8 29 1.2 4 4.2
265 105 1.2 31 1.3 1 1
266 128 1.4 42 1.7 1 1
267 116 1.3 30 1.2 1 1
268 126 1.4 31 1.3 2 2.1
269 155 1.7 55 2.3 1 1
270 200 2.2 60 2.5 1 1
271 183 2 55 2.3 1 1
272 199 2.2 73 3 1 1
273 260 2.9 78 3.2 3 3.1
274 258 2.8 54 2.2 1 1
275 673 7.4 70 2.9 6 6.3
276 323 3.6 104 4.3 7 7.3
277 333 3.7 98 4.1 1 1
278 346 3.8 91 3.8 1 1
279 398 4.4 103 4.3 1 1
280 672 7.4 185 7.7 1 1
281 456 5 120 5 1 1
282 381 4.2 120 5 2 2.1
283 374 4.1 98 4.1 1 1
284 349 3.8 79 3.3 2 2.1
285 374 4.1 80 3.3 1 1
286 346 3.8 107 4.4 1 1
287 353 3.9 97 4 1 1
288 284 3.1 59 2.4 2 2.1
289 213 2.3 66 2.7 1 1
290 196 2.2 67 2.8 1 1
291 172 1.9 19 0.8 2 2.1
292 100 1.1 40 1.7 3 3.1
293 99 1.1 19 0.8 2 2.1
294 62 0.7 22 0.9 1 1
295 23 0.3 15 0.6 1 1
296 20 0.2 8 0.3 2 2.1
297 15 0.2 7 0.3 2 2.1
298 8 0.1 3 0.1 1 1
299 11 0.1 1 0 2 2.1
300 7 0.1 1 0 2 2.1

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?
Download the data
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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