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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
0334439W,5D
196439W,5D
28339W,2D

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

Day 0 previous births 1 previous birth 2 or more previous births
#% #% #%
245 4 0.1 1 0.1 1 1.2
246 6 0.2 0 0 0 0
247 3 0.1 2 0.2 1 1.2
248 9 0.3 2 0.2 1 1.2
249 4 0.1 1 0.1 0 0
250 11 0.3 1 0.1 0 0
251 6 0.2 3 0.3 1 1.2
252 14 0.4 4 0.4 1 1.2
253 14 0.4 2 0.2 0 0
254 12 0.4 7 0.7 1 1.2
255 17 0.5 4 0.4 1 1.2
256 9 0.3 4 0.4 1 1.2
257 10 0.3 9 0.9 1 1.2
258 20 0.6 2 0.2 1 1.2
259 21 0.6 9 0.9 1 1.2
260 19 0.6 2 0.2 1 1.2
261 25 0.7 5 0.5 4 4.8
262 46 1.4 5 0.5 3 3.6
263 38 1.1 9 0.9 1 1.2
264 31 0.9 12 1.2 3 3.6
265 49 1.5 9 0.9 2 2.4
266 47 1.4 12 1.2 1 1.2
267 45 1.3 12 1.2 1 1.2
268 39 1.2 11 1.1 2 2.4
269 70 2.1 19 2 4 4.8
270 77 2.3 22 2.3 1 1.2
271 78 2.3 28 2.9 1 1.2
272 76 2.3 32 3.3 1 1.2
273 106 3.2 33 3.4 1 1.2
274 97 2.9 19 2 3 3.6
275 95 2.8 33 3.4 2 2.4
276 127 3.8 47 4.9 2 2.4
277 127 3.8 43 4.5 3 3.6
278 144 4.3 40 4.1 1 1.2
279 171 5.1 42 4.4 1 1.2
280 176 5.3 56 5.8 1 1.2
281 186 5.6 41 4.3 1 1.2
282 150 4.5 56 5.8 1 1.2
283 143 4.3 37 3.8 3 3.6
284 151 4.5 29 3 2 2.4
285 135 4 37 3.8 1 1.2
286 125 3.7 47 4.9 3 3.6
287 135 4 39 4 4 4.8
288 101 3 23 2.4 1 1.2
289 79 2.4 23 2.4 1 1.2
290 70 2.1 29 3 1 1.2
291 68 2 8 0.8 1 1.2
292 43 1.3 23 2.4 2 2.4
293 45 1.3 5 0.5 2 2.4
294 30 0.9 11 1.1 5 6
295 9 0.3 7 0.7 1 1.2
296 11 0.3 4 0.4 1 1.2
297 5 0.1 1 0.1 1 1.2
298 4 0.1 1 0.1 1 1.2
299 5 0.1 0 0 1 1.2
300 0 0 0 0 0 0

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
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