mjhjmtu

I am an elder god beyond time and space who mortals refer to as Shub-Niggurath. I have decided to travel to the middle ages of earth to design an experiment on the human race by opening a breach between the mortal realm and a hell-like dimension. This has filled the atmosphere with miasma-like radiation. These negative energies have caused an adverse reaction on the environment, mutating plants and wildlife. Although humans have remained unchanged, one way it has affected them is through its birth rates.

Unborn fetuses are especially vulnerable to this radiation, causing them to mutate into creatures known as dark young. This disease results in the growth of extra limbs, eyes, organs, and other horrific mutations. The creatures rip their way out of the mother, killing her in the process. They infest the earth and are extremely dangerous to anything they come across, growing bigger as time passes.

In this world midwifery has become an essential part of society. These are witches who practice a kind of magic called biomancy in order to battle adverse mutations in children. This magic takes the form of herbs, charms, rituals, and various ingredients to allow for a successful birth, or at least save the mother in the process of aborting the child. Although these methods have been known to work, dying in childbirth is very common and has had a harmful effect on civilized society. This radiation increases or decreases due to periodic cycles, with death rates rising and falling through the years.

What percentage of maternal death rates can humanity afford without endangering them as a species? And how would these circumstances affect reproductive strategies?

https://en.wikipedia.org/wiki/Sub-replacement_fertility

Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the older, previous one in a given area. In developed countries sub-replacement fertility is any rate below approximately 2.1 children born per woman, but the threshold can be as high as 3.4 in some developing countries because of higher mortality rates.

Since the Middle Ages was hardly developed by modern standards, the average woman would have to bear 3.4 children. (If you're math challenged, on average, every 10 women would have to bear 34 children.)

Ten women needing to bear 34 children leads to a 10/34 = 29.42% chance of each mother dying during each pregnancy and still being able to produce the required 34 children. That translates to a maternal death rate which is the staggering 29412/100000 live births.

According to this site, which refers to CIA world factbook, the country with highest maternal death rate is Sierra Leone, with 1360 deaths/100000
live births.

The same site also give for Sierra Leone a population growth rate of 2.38% and a birth rate of 36.3 births/1000 population.

For comparison, Netherlands has a maternal death rate of 7/100000 live births, a growth rate of 0.39% and a birth rate of 10.9/1000 population.

Based on these number that death rate is not detrimental to growth, as long as many new kids are born.

And that strategy would also apply to your world, with the addition of a careful monitoring of the pregnant women, to eliminate any problem before it becomes too big to handle.

As I understand your question in the world you create there would be 100% mortality without a "biomancer" to help.
This would create a strong boundary for casual sex and so this would reflect in strong social taboos.
All women would be "required" to have children as a first duty.
Pregnancy would be a much a more fearful experience for new mothers than it was in the middle ages.
The "biomancer" would be directly linked to the success or failure and would need some "protection" from failure. This may come in the form of physical power, after all if things go wrong a "dark young" needs dealing with or "rank and title" within the community they serve and likely both.

While your direct question is about the maternal death rate that maybe sustainable by the population, the world that evolves from your premise is a great deal more dangerous than mankind has endured in more than 6,000 years. The lurking "dark young" who are 'extremely dangerous to everything' would inflict their own "mortality rate" on whole populations unless adequately defended.

The birth rate required from a woman my actually lead to further mortality due to excessive biological demand on the body.

To give the humans a chance the "biomancer" would need to be as effective as the midwives of the middle ages despite the changes in the world. Also note that the middle ages where rife with plagues, without cures. This may give you some "linkage" for your storyline and more understanding of the effects your story world changes might have.

High maternal death rates are one of the fastest ways to accelerate evolution of a species.

White skin is a good example. The white skin of Europeans is now thought to have rapidly spread through the human populations there, starting 8000 years ago and probably saturating the populace within 1000 years.

Why would white skin be so advantageous? The colder and darker it is, the less sunlight on skin happens, and so it becomes harder to make vitamin D. White skin facilitates the production of vitamin D.

If you grow up without enough vitamin D, you get a bone condition called rickets. Girls with rickets had deformed pelvises. They grew up and could get pregnant, but the baby could not come out and the woman and baby would die. This was still a problem through the late 1800s.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2681281/.

At the turn of the 20th century, rickets ran rampant in the newly
industrialized cities of Europe and North America, and rachitic pelvis
was a common cause of death in childbirth (1). Cesarean sections
became established, in part, to manage this condition

If you cannot deliver a baby you have zero fitness and are a genetic dead end. If you can successfully deliver a baby in these circumstances there will be more resources for that baby because others in its generation were not born. In the next generation there will be more children of light skinned mothers who could make enough vitamin D to have normal pelvises. That gene will spread fast.

So too with your magic badness. If there is a woman who is somehow resistant, many of the children in the next generation will be hers. Maybe in a situation where mutations are more common this could help? How humans might evolve resistance to magical badness and what that would look like is up to the author, but it could be cool.

More women dying in childbirth changes society in many ways for sure, but it also leads to fewer children. You also have fewer children because many (most?) are either aborted or are born as "dark young." It's unclear from your question how many fully human newborns survive (meaning babies that will stay with the community and grow up to reproduce).

If the radiation waxes and wanes in cycles, there would be a lot more family planning. Really good birth control and early abortion (these things happened in pre-industrial societies where women had status and weren't murdered for providing these services...obviously not as effective as modern day hormones and barriers, but can be better than you think). There would be encouragement to have as many pregnancies as possible to keep the population from tanking, but only during "safer" time periods. Ordinary childbirth complications would also be reduced because midwives were valued, experienced, and well trained.

Is there a method to determine more or less of the radiation besides just looking at pregnancies and birth outcomes?