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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
0386439W,5D
1112039W,5D
28839W,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 7 0.2 1 0.1 1 1.1
246 11 0.3 0 0 2 2.3
247 3 0.1 2 0.2 1 1.1
248 11 0.3 2 0.2 1 1.1
249 6 0.2 2 0.2 1 1.1
250 11 0.3 1 0.1 1 1.1
251 8 0.2 3 0.3 1 1.1
252 17 0.4 5 0.4 0 0
253 15 0.4 2 0.2 0 0
254 13 0.3 7 0.6 1 1.1
255 22 0.6 4 0.4 1 1.1
256 13 0.3 7 0.6 1 1.1
257 12 0.3 11 1 1 1.1
258 21 0.5 2 0.2 1 1.1
259 23 0.6 11 1 1 1.1
260 24 0.6 4 0.4 1 1.1
261 24 0.6 6 0.5 4 4.5
262 50 1.3 6 0.5 3 3.4
263 45 1.2 14 1.3 1 1.1
264 36 0.9 13 1.2 2 2.3
265 57 1.5 13 1.2 2 2.3
266 57 1.5 18 1.6 1 1.1
267 52 1.3 14 1.3 1 1.1
268 46 1.2 15 1.3 2 2.3
269 79 2 26 2.3 5 5.7
270 95 2.5 26 2.3 1 1.1
271 90 2.3 30 2.7 1 1.1
272 93 2.4 37 3.3 1 1.1
273 120 3.1 37 3.3 1 1.1
274 119 3.1 22 2 3 3.4
275 111 2.9 32 2.9 4 4.5
276 147 3.8 57 5.1 2 2.3
277 147 3.8 48 4.3 1 1.1
278 168 4.3 47 4.2 1 1.1
279 199 5.2 51 4.6 1 1.1
280 191 4.9 60 5.4 1 1.1
281 206 5.3 48 4.3 1 1.1
282 174 4.5 63 5.6 1 1.1
283 173 4.5 42 3.8 3 3.4
284 167 4.3 35 3.1 2 2.3
285 165 4.3 45 4 1 1.1
286 143 3.7 53 4.7 3 3.4
287 154 4 47 4.2 4 4.5
288 112 2.9 26 2.3 1 1.1
289 91 2.4 26 2.3 1 1.1
290 81 2.1 31 2.8 1 1.1
291 82 2.1 10 0.9 1 1.1
292 47 1.2 25 2.2 2 2.3
293 49 1.3 7 0.6 1 1.1
294 30 0.8 11 1 7 8
295 10 0.3 7 0.6 1 1.1
296 13 0.3 4 0.4 2 2.3
297 7 0.2 1 0.1 1 1.1
298 5 0.1 2 0.2 1 1.1
299 5 0.1 0 0 1 1.1
300 1 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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