A distinction between human life and the presence of a human soul is not philosophically tenable. Or at least not insofar as many who put it forward attempt to use St. Thomas Aquinas as support for their position.
It is true that St. Thomas, following the received embryology of his day, did not believe that the ensoulment of an immortal, rational soul happened until some few months after conception. What is never mentioned is that, given St. Thomas' understanding of the relationship between body and soul, this means that there was no human life until after the ensoulment of an immortal, rational soul.
The soul is the substantial form of the body. This means that the soul makes the body be the kind of being that it is and grants to it all the powers that it possesses. Now, a human being is a rational animal. This means that it possesses a rational soul, from which stem its powers of intellect and will. But, since the soul is the substantial form of the body, this rational soul is also the origin of a man's vegetative and animal powers, i.e. the powers of life, reproduction, growth, nutrition, sensation and local motion.
Now, in the received embryology of the day, the being in the womb after conception first possessed only a vegetative soul. Thus it possessed only the powers of vegetative life, nutrition, growth. After some development this vegetative soul was replaced by an animal soul. Thus the being no longer had simply vegetative life, but animal life. Thus, in addition to the powers of nutrition and growth it now began to develop the powers of sensation and local motion, as seen through the development of sense organs and limbs. Finally, when this animal development had reached a fitting stage, God would infuse the being with an immortal, rational soul. Animal life would now be replaced by human life, and the rational soul would be the origin not only of the powers of nutrition, growth, sensation and local motion, but also of the powers of intellect and will.
As this brief sketch hopefully makes clear, there was no human life before the infusion of an immortal, rational, human soul. For to have a human life is to be a human being, to be a human being is to be a rational animal, and to be a rational animal is have an immortal, rational soul.
Thus those who would use St. Thomas as cover for their pro-abortion positions demonstrate only that they know nothing of St. Thomas. For in St. Thomas there can only be human life were matter is informed by an immortal, rational soul. If it can be demonstrated that human life begins at conception, then, for St. Thomas, it would be demonstrated that a human soul is infused by God and present at the moment of conception. To say otherwise would be to imply a duality between body and soul that is entirely foreign to the thought of St. Thomas.
An interesting note: The Council of Vienna formally declared that "whoever shall obstinately presume in turn to assert, define, or hold that the rational or intellective soul is not the form of the human body in itself and essentially must be regarded as a heretic" (Denzinger, 30th ed., no. 481). The conclusion that can be drawn from this pronouncement and the brief philosophical sketch given above I will leave to the reader.