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?
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).