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
0830839W,5D
1229839W,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 0.3 1 1
246 17 0.2 1 0 3 3.1
247 12 0.1 6 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 0.3 1 1
251 17 0.2 3 0.1 3 3.1
252 34 0.4 9 0.4 1 1
253 26 0.3 3 0.1 1 1
254 24 0.3 12 0.5 1 1
255 35 0.4 8 0.3 1 1
256 23 0.3 9 0.4 2 2.1
257 27 0.3 15 0.7 1 1
258 41 0.5 12 0.5 1 1
259 62 0.7 21 0.9 2 2.1
260 55 0.7 16 0.7 1 1
261 56 0.7 18 0.8 4 4.2
262 76 0.9 15 0.7 1 1
263 83 1 24 1 3 3.1
264 73 0.9 29 1.3 4 4.2
265 105 1.3 27 1.2 1 1
266 124 1.5 38 1.7 1 1
267 115 1.4 30 1.3 1 1
268 125 1.5 31 1.3 2 2.1
269 152 1.8 55 2.4 1 1
270 199 2.4 59 2.6 1 1
271 179 2.2 55 2.4 1 1
272 196 2.4 72 3.1 1 1
273 260 3.1 76 3.3 3 3.1
274 249 3 54 2.3 1 1
275 250 3 69 3 6 6.3
276 312 3.8 102 4.4 7 7.3
277 330 4 93 4 1 1
278 339 4.1 91 4 1 1
279 392 4.7 98 4.3 1 1
280 487 5.9 131 5.7 1 1
281 448 5.4 115 5 1 1
282 371 4.5 117 5.1 2 2.1
283 365 4.4 92 4 1 1
284 339 4.1 79 3.4 2 2.1
285 369 4.4 79 3.4 1 1
286 342 4.1 103 4.5 1 1
287 346 4.2 90 3.9 1 1
288 276 3.3 59 2.6 2 2.1
289 208 2.5 63 2.7 1 1
290 189 2.3 66 2.9 1 1
291 167 2 18 0.8 2 2.1
292 98 1.2 38 1.7 3 3.1
293 98 1.2 19 0.8 2 2.1
294 61 0.7 21 0.9 1 1
295 21 0.3 15 0.7 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 9 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
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