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:
- The average first time mom had her baby at 39W,5D. 54.7% of first time moms delivered their babies on or before their due date. This is based on 8660 first time moms who filled out the survey.
- The average second time mom had her baby at 39W,5D. 56.2% of second time moms delivered their babies on or before their due date. This is based on 2437 second time moms who filled out the survey.
- For mothers with more than one previous birth, the delivery was at 39W,0D. 59.5% of these moms delivered their babies on or before their due date. This is based on 95 births.
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 births | Average Arrival |
|---|
| 0 | 8660 | 39W,5D |
| 1 | 2437 | 39W,5D |
| 2 | 95 | 39W,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 |
16 |
0.2 |
6 |
0.2 |
1 |
1.1 |
| 246 |
19 |
0.2 |
1 |
0 |
3 |
3.2 |
| 247 |
12 |
0.1 |
7 |
0.3 |
1 |
1.1 |
| 248 |
18 |
0.2 |
3 |
0.1 |
1 |
1.1 |
| 249 |
18 |
0.2 |
4 |
0.2 |
2 |
2.1 |
| 250 |
17 |
0.2 |
6 |
0.2 |
1 |
1.1 |
| 251 |
19 |
0.2 |
3 |
0.1 |
3 |
3.2 |
| 252 |
35 |
0.4 |
10 |
0.4 |
1 |
1.1 |
| 253 |
26 |
0.3 |
4 |
0.2 |
1 |
1.1 |
| 254 |
25 |
0.3 |
14 |
0.6 |
1 |
1.1 |
| 255 |
35 |
0.4 |
8 |
0.3 |
1 |
1.1 |
| 256 |
24 |
0.3 |
11 |
0.5 |
2 |
2.1 |
| 257 |
30 |
0.3 |
15 |
0.6 |
1 |
1.1 |
| 258 |
43 |
0.5 |
12 |
0.5 |
1 |
1.1 |
| 259 |
63 |
0.7 |
21 |
0.9 |
2 |
2.1 |
| 260 |
59 |
0.7 |
17 |
0.7 |
1 |
1.1 |
| 261 |
59 |
0.7 |
18 |
0.7 |
4 |
4.2 |
| 262 |
78 |
0.9 |
15 |
0.6 |
1 |
1.1 |
| 263 |
93 |
1.1 |
24 |
1 |
3 |
3.2 |
| 264 |
78 |
0.9 |
30 |
1.2 |
4 |
4.2 |
| 265 |
108 |
1.2 |
32 |
1.3 |
1 |
1.1 |
| 266 |
135 |
1.6 |
42 |
1.7 |
1 |
1.1 |
| 267 |
122 |
1.4 |
35 |
1.4 |
1 |
1.1 |
| 268 |
129 |
1.5 |
33 |
1.4 |
2 |
2.1 |
| 269 |
158 |
1.8 |
55 |
2.3 |
1 |
1.1 |
| 270 |
205 |
2.4 |
63 |
2.6 |
1 |
1.1 |
| 271 |
189 |
2.2 |
55 |
2.3 |
1 |
1.1 |
| 272 |
206 |
2.4 |
74 |
3 |
1 |
1.1 |
| 273 |
265 |
3.1 |
82 |
3.4 |
3 |
3.2 |
| 274 |
264 |
3 |
56 |
2.3 |
1 |
1.1 |
| 275 |
259 |
3 |
71 |
2.9 |
6 |
6.3 |
| 276 |
325 |
3.8 |
108 |
4.4 |
8 |
8.4 |
| 277 |
339 |
3.9 |
101 |
4.1 |
1 |
1.1 |
| 278 |
358 |
4.1 |
92 |
3.8 |
1 |
1.1 |
| 279 |
405 |
4.7 |
108 |
4.4 |
1 |
1.1 |
| 280 |
503 |
5.8 |
133 |
5.5 |
1 |
1.1 |
| 281 |
462 |
5.3 |
124 |
5.1 |
1 |
1.1 |
| 282 |
384 |
4.4 |
127 |
5.2 |
1 |
1.1 |
| 283 |
384 |
4.4 |
100 |
4.1 |
1 |
1.1 |
| 284 |
357 |
4.1 |
81 |
3.3 |
1 |
1.1 |
| 285 |
377 |
4.4 |
83 |
3.4 |
1 |
1.1 |
| 286 |
349 |
4 |
109 |
4.5 |
1 |
1.1 |
| 287 |
356 |
4.1 |
105 |
4.3 |
1 |
1.1 |
| 288 |
291 |
3.4 |
59 |
2.4 |
2 |
2.1 |
| 289 |
220 |
2.5 |
68 |
2.8 |
1 |
1.1 |
| 290 |
202 |
2.3 |
69 |
2.8 |
1 |
1.1 |
| 291 |
177 |
2 |
19 |
0.8 |
2 |
2.1 |
| 292 |
105 |
1.2 |
41 |
1.7 |
3 |
3.2 |
| 293 |
102 |
1.2 |
21 |
0.9 |
2 |
2.1 |
| 294 |
62 |
0.7 |
23 |
0.9 |
1 |
1.1 |
| 295 |
24 |
0.3 |
16 |
0.7 |
1 |
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 |
1.1 |
| 299 |
11 |
0.1 |
1 |
0 |
2 |
2.1 |
| 300 |
7 |
0.1 |
1 |
0 |
2 |
2.1 |
