No, no matter how awful his Aristotelianism is, Thomas wouldn’t have countenanced abortion. The problem is deeper than that, in that he is a temporary Apollinarian. Here is the problem:
Do babies in the womb who are younger than 40 days have a rational soul?
Thomas said no. That means that Baby Jesus was an Apollinarian for a few weeks. He writes,
Therefore the vegetative soul, which comes first, when the embryo lives the life of a plant, is corrupted, and is succeeded by a more perfect soul which is both nutritive and sensitive, and then the embryo lives an animal life; and when this is corrupted it is succeeded by the rational soul introduced from without: although the preceding souls were produced by the virtue in the semen. (Summa Contra Gentiles, II, 89)
The Thomist has a possible response: the vegetative soul is not actually a different soul, so Baby Jesus has a soul, of sorts. But if you are a Reformed Christian, this answer won’t do, since Westminster says Baby Jesus took on a rational soul.
Let’s say it a different way: can you terminate the life of bodies with vegetative souls? Decent human beings would say no. Are they people? It’s harder for them to say yes. Paul Helm, an otherwise excellent theologian, writes (Human Nature from Calvin to Edwards)
The product of a human father and mother that was solely in a vegetative state would not be a person in the way in which someone who was born without legs but was otherwise “normal” would be a person, as one who is incomplete, handicapped, or “challenged.” A person who is born in a persistent vegetative state (PVS) would be entitled to be cared for and treated decently in life and then in death; that person would have human parents and would be possessed of a characteristically human form and not be a member of any other species; but that one would not be a person, or fully a person nor have the potential to be one. What does the one born in a PVS ab initio lack? We might say that he or she–for this one in a PVS would have a biological gender–lacks a mind. If that one could, with help, come to think and to feel sensations and make judgments and perform actions and feel emotions, going beyond the feeling of physical sensations of pain and pleasure, then we’d be correct to call that being a person.
This is where Thomism leads.