If you pay careful attention to article 4, objection 2, you will note that St. Thomas takes for granted that the first stage of normal fetal development does not involve a human person with a rational soul, but rather material that can develop into the flesh of a human person.
More-recent observations of prenatal life have put this presumption of St. Thomas’ into a different light, to say the least. Our current state of knowledge makes the traditional concept of “quickening”–at some mid-point in pregnancy–meaningless, as far as identifying the presence of a distinct human person.
If you remember Question 4, article 1, you know that St. Thomas holds that it is precisely the rationality and intellectuality of human nature that makes it ‘assumable’ by the Word of God.
Therefore, St. Thomas has to argue that the infinite divine power perfected the nascent physical matter of Christ’s flesh instantaneously, at the moment of conception, by a miracle.
For most of us pro-life Catholics, this is actually the way we think of every pregnancy, given what we now know about pre-natal development.
But it does not help the pro-life cause for us to overstate our case. So, to clarify, when it comes to official Church teaching (which St. Thomas did uphold):
The Church teaches that intentional abortion is wrong. She does not teach the precise metaphysical why.
The victim of the act of violence involved in an intentional abortion may have a rational soul, but we don’t know for sure.
That said, even the mere possibility that the victim of the act of violence has a rational soul; and the certainty that the victim is developing into a rational human being the same way we all did, when we were at that stage–either one is enough to stay the hand that would do violence.
The Church does not have official teaching about when the soul begins to animate the flesh in the womb. But the moral obligation not to do violence remains clear anyway.