Alzheimer's is very different than Braindead. Alzheimers means you can't express any coherent thought. Braindead is that higher function is not even measurable or non-existent.
If Roe v Wade were to be re-evaluated today, it is possible that the science could support that by definition the fetus is not human or classified as alive at least until the 26th week and possibly beyond that.
What is human and how are different from other primates. According to life sciences, the main distinctions are self-awareness, abstract thought, a promiscuous combination of ideas, and generative computation. None of these attributes can exist in a fetus before the 26th week and is not measurable until after birth.
That is why this is such a complex subject. What do you use to determine life, when is a fetus human, when is a fetus a sentient being? Should this be determined purely on science, morality, religion, or ethics?