A fetus doesn't breathe, actually, not that it matters.

It does actually (sort of). Fetus (feti/fetod?) begin practicing breathing as early as 10 weeks. This starts out as the contractions in muscles that will be required after birth. This continues all the way up until birth at which point the lungs are expanding and contracting (ie filling with fluid and draining). When the baby is born, its chest muscles will contract to push out the fluid and begin utilizing oxygen from the air.

As for sentience, a fetus will not measurably have that property should adult animals lack it.

So killing something non-sentient is OK? I had a buddy in high school whose brother was born with massive chunks of his brain failing to form. He as expected to die within a few days but ended up surviving 13+ years. He was on massive morphine injections all the time and had 0 development going on. He was unable to recognize people. By all accounts, this is less developed than many fetus are (if you only look at what they are capable of thinking). We have evidence that dreaming begins as early as 13 weeks in pregnancy, and yet this kid was unable to achieve rem sleep (when you dream). By all the laws we have today, if the Mother had simply smothered the child one night, she would have been arrested and sent to jail. Why is this any different? The kid was not self aware, was in constant torment and had 0 chance at survival. What makes the fetus sub human? (I would tend to support a caretakers right to end the life of someone terminal, so this is just an example, not my belief)

Finally, you mentioned that adult animals lack sentience, but if you research that, you will find that the distinction is much less clear than previously thought. Animals have been shown to communicate identities through sound, use tools, and solve relatively complex problems.