The Horror of Pregnant Women
So, what's up with pregnant women?
I have nothing too outlandish to offer, but I'd make a couple observations.
First, unless one works in the medical profession, and especially in trauma, birth is one of the few times you're going to see that much blood and other fluids flying around (and yes, they are airborn for parts of the process), or when you'll see so much excruciating pain, and pain in someone you know and like. And all of this is conflicted, as you're supposed to be happy at a birth, yet the woman is screaming in agony. So a birth scene and all the conflicted and scary feelings that go along with it is more real to us than a scene of torture or dismemberment, since for most all of us, those exist solely in our imagination.
Of course, usually in horror movies, this very natural, beautiful scene goes horribly awry. This increases the terror, by presenting it as a perversion of something beautiful. This is increased even more, because all of us who have witnessed a birth have memories of how tense and anxious we were during the process, with all our fears of what "normal" things can go wrong - never mind aliens, demons, or zombies bursting from the gal's guts.
And that anxiety points to one final observation. Even before the birth, even without the horrible twist that ends in monstrosity and death, a pregnant woman is somehow a liminal and hybrid being - two people at once, fully alive and potentially alive, but with something that might kill the host in the process, and all of this while being filled with hormones that can send her into uncontrolled furies (designed, probably, to help her defend her young from adult males who would be inclined to kill and eat them, the way lions do). I think a pregnant woman really can only be an object of numinous awe at the mystery and fragility of life.
And good horror, at least, traffics in the same intuitions of numinous awe - at life, death, beauty and pain.