A fetus is sentient. It is not sapient. An embryo or zygote is neither. Furthermore a non sentient human, like a coma victim, is a person.
A fetus becomes sentient in the 3rd trimester, certainly. But the phrase 'a fetus is sentient', as a catchall, is false. You'll note that my statement was
if the fetus never becomes sentient.
As well, a coma victim is functionally different from a fetus in that it certainly
was sentient. It's qualitatively different, in the 'having had sentience' is a feature a fetus does not have.
Once sentience occurs, there's a manifest
shifting in the nature of the organism. The body now
belongs to the sentient organism, and this ownership extends back in time to his constituents. No one thinks a sperm is morally significant, but if you were to poison the sperm that created me, it's clearly an assault on
me. The ownership extends forwards and backwards, but only once my status as a person is actualized. You may poison sperm all you want, if they're disposed of.
So, the coma patient's body
belongs to the coma patient, triggered when that patient had originally become sentient. We let people announce whether
they will be DNRed, and we let them give commands on the disposal of their corpse.
A fetus
cannot be compared to a coma patient, they're qualitatively different.