Oxygen is supplied by the mother, but the fetus, submerged in amniotic fluid is not breathing on it's own. Keep in mind, that when a baby is born, God breathes into it's **nostrils** the breath of life.
With a few exceptions, I am not ok with any kind of abortion, any more than I am ok with someone having their leg cut off due to gangrene, but sometimes there are overriding circumstances. For instance, if my ten-year-old daughter was pregnant due to being raped, I would not encourage her in any way, to go through with the pregnancy. Similar to the Levitical law that demanded that the fetus, the product of adultery must be aborted. And most abortions are connected with some sort of immoral relationship. Bible definitely approves of such measures.