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?