For RC's the Ubbar's life always takes precedence
For Feminists, the Mom's life always takes precedence
Black and White
Halachah has a more nuanced view - pithily summarized by Kitzur SA 184:11 [following SA Ch"M 425:2]
It seems it's all based upon hazzakah.
Until Birth, the Ubbar has no Hezkat Hayyim, therefore the Mom's life would always takes precedence
After Birth - namely when the head emerges - it's "even Steven" and we actively attack neither
And this is the "Nature of the World" - Seems a Natural to me.