Length of pregnancy for first time vs. second & third time moms

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:

The average first time mom had her baby at 39W,5D. 58.6% of first time moms delivered their babies on or before their due date. This is based on 9321 first time moms who filled out the survey.

The average second time mom had her baby at 39W,5D. 57.8% of second time moms delivered their babies on or before their due date. This is based on 2455 second time moms who filled out the survey.

For mothers with more than one previous birth, the delivery was at 39W,0D. 60.8% of these moms delivered their babies on or before their due date. This is based on 96 births.

Average length of pregnancy, by number of subsequent previous births

# of previous births # of births Average Arrival 0 9321 39W,5D 1 2455 39W,5D 2 96 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 15 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 10 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 0.2 11 0.4 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 17 0.7 1 1 261 58 0.6 18 0.7 4 4.2 262 77 0.8 15 0.6 1 1 263 91 1 24 1 3 3.1 264 75 0.8 30 1.2 4 4.2 265 107 1.1 31 1.3 1 1 266 128 1.4 42 1.7 1 1 267 116 1.2 30 1.2 1 1 268 127 1.4 32 1.3 2 2.1 269 155 1.7 55 2.2 1 1 270 200 2.1 61 2.5 1 1 271 184 2 55 2.2 1 1 272 199 2.1 73 3 1 1 273 260 2.8 78 3.2 3 3.1 274 259 2.8 54 2.2 1 1 275 833 8.9 70 2.9 6 6.3 276 324 3.5 106 4.3 7 7.3 277 333 3.6 98 4 1 1 278 347 3.7 91 3.7 1 1 279 400 4.3 103 4.2 1 1 280 736 7.9 212 8.6 1 1 281 456 4.9 120 4.9 1 1 282 381 4.1 121 4.9 2 2.1 283 374 4 100 4.1 1 1 284 350 3.8 79 3.2 2 2.1 285 376 4 81 3.3 1 1 286 347 3.7 107 4.4 1 1 287 355 3.8 99 4 1 1 288 285 3.1 59 2.4 2 2.1 289 214 2.3 66 2.7 1 1 290 197 2.1 67 2.7 1 1 291 172 1.8 19 0.8 2 2.1 292 101 1.1 40 1.6 3 3.1 293 99 1.1 19 0.8 2 2.1 294 62 0.7 22 0.9 1 1 295 23 0.2 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

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.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.