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Historically, multiple births (twins, triplets, etc.) are less likely to survive under circumstances of high infant mortality, what might change that so at least one tends to survive?

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  • $\begingroup$ You are asking about chances in the body of your question and about how to increase them in the title. Choose one. There of course are many solutions. You should maybe give a broad direction you want to go in and if you want a solution that exclusively makes one sibling survive and not increase the total survival rate of any birth and so on $\endgroup$ – Raditz_35 Jan 30 '18 at 11:42
  • $\begingroup$ Switch the species. Cats and dogs and pigs have a much higher survival rate for multiple births. $\endgroup$ – Real Subtle Jan 30 '18 at 12:16
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There are two factors that tend to give multiple births higher mortality:

  • More stress on the mother, which, in low hygiene environment, directly translates in childbirth problems for both mother and children.
  • Tendency to have "early birth", which translates in newborn problems if an incubator is not available.

If you want to give multiple pregnancies a better chance to result in healthy children you can:

  • have the "normal" children be smaller with respect to mother (I mean: either have normal newborn well below 2kg or substantially enlarge mother belly).
  • introduce a mechanism where fetuses "fight" for space while in the womb, so that result will be some "very early" births (presumably miscarriages) and a final "normal birth" of the "winning" one.
  • introduce some post-partum device able to protect premature (e.g.: some kind of skin pouch as found in marsupials).
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  • $\begingroup$ If you are willing to make the protagonists non-human, then there are plenty examples of animal species giving birth to multiple living babies and most of them surviving (under good conditions). This includes many domesticated animals (dogs, rabbits, pigs...) and hemerophiles (cats, rodents...). In fact the majority of mammals procreate that way. In this regard humans are an exception, not the rule. $\endgroup$ – Philipp Jan 30 '18 at 12:13
  • $\begingroup$ Let's assume that they're either related to humans or rather close to them in physiology, what then? $\endgroup$ – Preg-Fan Jan 30 '18 at 14:34
  • $\begingroup$ @Preg-Fan: It's difficult to answer that. There are "good reasons" why things are as they are, so good Evolution was forced to find a way to ensure multiple birth is a rare exception. In order to revert this you must change something substantial (risking other problems). I stated what are (IMHO) the minimal requirements (each one of them would be enough). I can't know what You are inclined to accept, but be assured some non-trivial changes will be needed, otherwise Evolution would (most likely) have found a way to "harden" women. $\endgroup$ – ZioByte Jan 31 '18 at 9:27

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