To me the most cut and dried pro abortion arguments are in cases of anencephaly - fetuses identified to be growing without brains or even without heads.

In both cases (but especially, anencephaly), the onus is infuriatingly on the mother forced to gestate and give birth at/near term to a child destined to die. (Some have suggested not only giving birth to such infants purely for the benefit of harvesting its organs, but even deliberately creating such fetuses explicitly for that purpose, even ex utero if science ever permits.)