# How long do humans gestate? **URL:** \ **Category:** Factual Questions\ **Created:** [August 22, 2009, 7:37pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337 "2009-08-22T19:37:59Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [August 22, 2009, 7:37pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/1 "2009-08-22T19:37:59Z")
I know, I know, nine months, but that’s got to be a rough approximation derived at a fairly early point of estimating. Have we gotten any closer in recent years to computing a more accurate estimate? Of course any estimate is going to be plus/minus a certain number of days, but what’s the median number? 270 days +/-10? 272 +/- 12? 267 +/-5? How precise have we gotten in arriving at a figure? I’m writing a story in which the baby’s birth is known to have started in early September, and the parents speculate a due date of early June, on the same same day of that month? Will their baby probably arrive earlier or later than exactly nine months after conception? ---
**Author:** ![picunurse](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@picunurse](https://boards.straightdope.com/u/picunurse)\ **Post date:** [August 22, 2009, 7:41pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/2 "2009-08-22T19:41:34Z")
40 weeks. ---
**Author:** ![janeslogin](https://avatars.discourse-cdn.com/v4/letter/j/df788c/32.png) [@janeslogin](https://boards.straightdope.com/u/janeslogin)\ **Post date:** [August 22, 2009, 7:52pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/3 "2009-08-22T19:52:03Z")
Most reliable I can find seems to be " … 280 days from the first day of the last menstrual period (LMP), or 266 days from ovulation (with a menstrual cycle of 28 days, ovulating on the fourteenth day)… " but the purpose of this article was to say that it might be longer because the assumption rather complicates things. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [August 23, 2009, 12:56am UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/4 "2009-08-23T00:56:58Z")
Isn’t the 40 weeks number based on the pregnancy starting at the last menstruation? In other words, by this definition, couldn’t a woman be pregnant for 2 weeks before she meets the father? And, wouldn’t 38 weeks be a better estimate of the average time between the intercourse that caused the pregnancy and the resulting birth? ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [August 23, 2009, 2:35am UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/5 "2009-08-23T02:35:26Z")
> [@pseudotriton\_ruber\_ruber](#): > > I know, I know, nine months, but that’s got to be a rough approximation derived at a fairly early point of estimating. Have we gotten any closer in recent years to computing a more accurate estimate? Of course any estimate is going to be plus/minus a certain number of days, but what’s the median number? 270 days +/-10? 272 +/- 12? 267 +/-5? How precise have we gotten in arriving at a figure? > > I’m writing a story in which the baby’s birth is known to have started in early September, and the parents speculate a due date of early June, on the same same day of that month? Will their baby probably arrive earlier or later than exactly nine months after conception? I assume you mean the baby’s conception was in early September? 266 days of gestation is the commonest number you’ll see, I suspect. The parents could use the typical calculator shown here, which assumes regular periods and counts 280 days from the first day of the last menstrual period. [http://www.healthline.com/yodocontent/pregnancy/your-due-date.html](http://www.healthline.com/yodocontent/pregnancy/your-due-date.html) Gestational age would be a couple weeks less than that, of course. More typically, ultrasounds are used to estimate gestational age, especially if done fairly early. See the chart here, for instance: [http://www.brooksidepress.org/Products/Military\_OBGYN/Textbook/Pregnancy/estimating\_gestational\_age.htm](http://www.brooksidepress.org/Products/Military_OBGYN/Textbook/Pregnancy/estimating_gestational_age.htm) ---
**Author:** ![Rhythmdvl](https://avatars.discourse-cdn.com/v4/letter/r/85f322/32.png) [@Rhythmdvl](https://boards.straightdope.com/u/Rhythmdvl)\ **Post date:** [August 23, 2009, 3:11am UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/6 "2009-08-23T03:11:35Z")
what if you know the _exact_ date of conception. Then add 266 to get the average delivery date? ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [August 23, 2009, 8:47am UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/7 "2009-08-23T08:47:07Z")
> [@Chief\_Pedant](#): > > I assume you mean the baby’s conception was in early September? Well, yes. :smack: I don’t mean that contractions started in September and went on until June. I’m talking about a circumstance where a couple has only only had sex one time in September, and they know exactly which date that was in September, so they’re sure that was the date they conceived on. In such an instance, how many days later could they expect the birth to be most probable? ---
**Author:** ![Aspidistra](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aspidistra/32/3894_2.png) [@Aspidistra](https://boards.straightdope.com/u/Aspidistra)\ **Post date:** [August 23, 2009, 9:12am UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/8 "2009-08-23T09:12:06Z")
If it’s a first pregnancy, it’s likely to be about [41 weeks](http://forums.obgyn.net/pregnancy-birth/P-B.0303/0704.html) (counting from the last period) ---
**Author:** ![pudytat72](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@pudytat72](https://boards.straightdope.com/u/pudytat72)\ **Post date:** [August 23, 2009, 12:54pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/9 "2009-08-23T12:54:56Z")
They may have had sex on day X, but that does not mean she got pregnant on day X. Sperm can live in the uterus and fallopian tubes of a female for as long as 5 days (2-3 days is more typical). If the female ovary released the egg on day X or 2-3 days before day X, then conception could occur on day X. If the egg came out on day X plus 2, then conception could not occur on day X. So you may know when sex happened, but you don’t know when the egg and the sperm finally got together. Too many other variables that medical science does not fully know about to say that a person will definitely have a baby on day X plus 265. But in literature, you can make your own rules, can’t you? ---
**Author:** ![pudytat72](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@pudytat72](https://boards.straightdope.com/u/pudytat72)\ **Post date:** [August 23, 2009, 1:04pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/10 "2009-08-23T13:04:37Z")
Anecdote does not equal data. . .If you count backward from my date of birth in August, it is 266 days to my mother’s birthday in November! Hmmm. . . ---
**Author:** ![Cazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cazzle/32/75_2.png) [@Cazzle](https://boards.straightdope.com/u/Cazzle)\ **Post date:** [August 23, 2009, 3:15pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/11 "2009-08-23T15:15:22Z")
My pregnancy was 272 days, but I was induced so it could (and probably would) have gone on longer. ---
**Author:** ![grimpixie](https://avatars.discourse-cdn.com/v4/letter/g/ecb155/32.png) [@grimpixie](https://boards.straightdope.com/u/grimpixie)\ **Post date:** [August 23, 2009, 4:18pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/12 "2009-08-23T16:18:30Z")
On our ante-natal course, we were told that the due date could vary from 36 to 42 weeks depending on which country in the world you were examined in. Grim ---
**Author:** ![ShibbOleth](https://avatars.discourse-cdn.com/v4/letter/s/848f3c/32.png) [@ShibbOleth](https://boards.straightdope.com/u/ShibbOleth)\ **Post date:** [August 23, 2009, 5:28pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/13 "2009-08-23T17:28:58Z")
Data from the USA is probably bad because of lawyers and malpractice suits: in other words, OB/GYNs are afraid to let a pregnancy go past a certain point because they’re afraid of a malpractice claim so they will induce or otherwise cause the baby to be delivered at a certain point. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [August 23, 2009, 6:29pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/14 "2009-08-23T18:29:48Z")
> [@grimpixie](#): > > On our ante-natal course, we were told that the due date could vary from 36 to 42 weeks depending on which country in the world you were examined in. > > Grim Yeah, in Spain they count 42 weeks. My parents know they made me on May 18 and I tried to be born on February 19th, but as has been mentioned, the actual date of conception may have been a few days later. ---
**Author:** ![Rhythmdvl](https://avatars.discourse-cdn.com/v4/letter/r/85f322/32.png) [@Rhythmdvl](https://boards.straightdope.com/u/Rhythmdvl)\ **Post date:** [August 23, 2009, 8:45pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/15 "2009-08-23T20:45:37Z")
> [@ShibbOleth](#): > > Data from the USA is probably bad because of lawyers and malpractice suits: in other words, OB/GYNs are afraid to let a pregnancy go past a certain point because they’re afraid of a malpractice claim so they will induce or otherwise cause the baby to be delivered at a certain point. Does any organization keep track of these statistics? If so, wouldn’t it be easy to account for cesareans or induced labour? To figure out which is most accurate; count-from-LMP, count-from-conception-event, count from ultrasound measurements, etc.? There’s also a host of wives’ tales that could easily be addressed/dispelled–it’s a boy? Oh, you’ll go over. It’s your first? Oh, you’ll deliver early. It’s your fourteenth? Oh, you must be very tired! ---
**Author:** ![StaudtCJ](https://avatars.discourse-cdn.com/v4/letter/s/90ced4/32.png) [@StaudtCJ](https://boards.straightdope.com/u/StaudtCJ)\ **Post date:** [August 23, 2009, 10:10pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/16 "2009-08-23T22:10:34Z")
> [@Rhythmdvl](#): > > what if you know the _exact_ date of conception. Then add 266 to get the average delivery date? Usually, you can’t know the exact date of conception, according to my midwife. You can know when you supposedly ovulated, and when you had intercourse or pregnancy inducing behavior; but you can’t know if sperm met egg a day later, 8 hours later, etc. That’s why they count from LMP or get an ultrasound to determine GA. ---
**Author:** ![wierdaaron](https://avatars.discourse-cdn.com/v4/letter/w/eada6e/32.png) [@wierdaaron](https://boards.straightdope.com/u/wierdaaron)\ **Post date:** [August 23, 2009, 10:15pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/17 "2009-08-23T22:15:04Z")
Some people think that humans, like kangaroos, do as much “gestating” after birth as before. Kangaroos are born gooey little useless blobs, and then grow up into functioning 'roos in their mother’s trademark pouches. Every other animal is fully functional within a day or two of being born. Humans, however, are functionally useless until around age 1. Our brains develop more in the first year than during the rest of our lives. So you could consider the first year of life to be continued gestation, except instead of being protected inside the womb, they have to be protected by the parents in a sort of metaphorical pouch of overpriced baby products. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [August 24, 2009, 8:07pm UTC](https://boards.straightdope.com/t/how-long-do-humans-gestate/507337/18 "2009-08-24T20:07:16Z")
> [@wierdaaron](#): > > […] Every other animal is fully functional within a day or two of being born. […] Gee - ever taken care of a kitten deserted at birth? They’re pretty far from fully functional.