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Bringing a human child to term generally takes place over a period of nine months. They are marked by different stages, from 1st to third, and are marked by different events and expectations for mother and child. While no two instances are the same, this process is generally similar enough to be predictable. Babies are generally in the womb for the same amount of time.

In an alternate world, women are regularly with child for more than a year, with the most extreme verified case being more than three. The "average pregnancy" lasts, about nine months, but comes with a "plus or minus six months" qualification. Because of gestational periods being more unpredictable, it becomes essentially impossible to determine when any given child will be born. Standard deviations are quite large, with any one individual having multiple several prrgnancies with various gestational times. This has no overall effect on the child itself.

What alterations to the biology of a species would lead to this development taking place?

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  • $\begingroup$ the "+-6 months" would lead to 3-month children, which totally isn't doable with the complex brain! $\endgroup$ – Trish Jan 28 at 16:52
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To form a "ready-to-be-born" baby one needs to follow a series of steps starting from the fertilized eggs.

Therefore in that sequence there is no room for variation, if you want to stay within a single species.

What can influence the duration of the pregnancy would be the capability for the woman to put the development on idle under certain circumstances. For example, a period of low food intake would stop the fetus development, so that the mother can more easily survive it. The development would then restart as soon as the food supply is back to normal.

As a consequence the apparent lenght of gestation would be different from mother to mother.

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  • $\begingroup$ "[The] length of gestation would be different from mother to mother": It already is different from mother to mother. In humans, pregnacy takes anywhere between 37 and 42 weeks. Only births before 37 weeks (premature or preterm) or later than 42 weeks (postmature or postterm) are considered abnormal. (Fun historical factoid: in the antiquity, the normal duration of a pregancy was considered to be 10 months. Why the popular perception shifted to 9 months in the modern age I don't know.) $\endgroup$ – AlexP Jan 19 at 13:17
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    $\begingroup$ @AlexP I wouldn't qualify a 10% oscillation as "wild". $\endgroup$ – L.Dutch - Reinstate Monica Jan 19 at 13:54
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    $\begingroup$ Most animals do this by either delaying fertilization with sperm storage (see link) or delay implantation (4 months in armadillo) with the embryo in a form of halted development while it is still very simple in structure usually the blastocyst stage. en.wikipedia.org/wiki/Female_sperm_storage $\endgroup$ – John Jan 19 at 14:55
  • $\begingroup$ List of mammalian gestation durations - the only one with a maximum that's upwards of twice the minimum is the (European mink : 38~76) mink : 40~75. Why? $\endgroup$ – Mazura Jan 19 at 21:48
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    $\begingroup$ @Mazura for mink, naturallynorthidaho.com/2016/01/… has an interesting explanation - "During breeding season, female mink mate with multiple males. Superfetation allows the female’s eggs to become fertilized with each mating male. Each fertilized egg develops to the blastocyst stage and then suspends development until the end of breeding season. Then all the embryos implant at the same time so they are all born at the same time." $\endgroup$ – Peteris Jan 20 at 0:27
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L.Dutch's answer would lead to a predictable result--less food, longer gestation.

Instead, there is a trace element the body needs that is in limited supply. Shortages are common enough that the body evolved to limit fetal growth to the supply of this trace element rather than suffer deficiency problems from it's lack.

Required trace elements have been found well into the 20th century (when IV nutrition became possible we saw previously unknown deficiency diseases) and there are still disputes about whether certain elements are required or not. Thus even a 19th century society would not know about these trace elements and not understand why some babies came faster than others.

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Human gestation periods were originally keyed, I imagine, to environmental factors, with an optimal strategy of conception in late spring, brooding through the winter months, and birth in early spring. This is a typical pattern for creatures on an estrous cycle, at any rate, because it gives offspring the most favorable conditions for survival in their first days. That predisposition likely carried over into the menstrual system that higher primates use.

So, one simple way to get wide variation in gestation (strictly through evolutionary adaptation) is to remove the seasonal variations that produce an environmental selective force. On a planet with a nearly circular orbit and negligible axial tilt, there would be no advantage to a gestation period of any certain length, so gestation length would be uncoupled from any time dependency. Natural variations in the length of gestation would accumulate in the genome, because no offspring would die off from being born 'out of season.' There would be some natural lower limit based on the minimal time in which a fetus can develop to a viable form, and some natural upper limit, likely based on the maximum size for a baby to pass through the birth canal. But within those limits there would be a bell-curve of 'natural' times based on genetic predispositions and other factors.

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