We must treat every life as sacred. The presence or absence of soul, or the problematic issue of what constitutes a humaness in some possibly non-viable collection of chromosomes or partial chromosomes is forever beyond the scope of our ability to know.
You must be careful here.
There’s a long and involved philosophical history in the question of ensoulment.
First, we need to clarify the meaning of “soul”. In Aristotle, “soul” is the “form” of “human being”. As such, the “soul” is the same in every human being.
“Soul” is not “person”. In fact, Aristotle did not have an adequate notion of “person”. In Aristotle, what constitutes individuality is the matter, not the person.
Now “person” denotes a singularity, a uniqueness. Whereas “soul” is the same for every human being, each “person” is radically different from every other “person”.
Later on, in Christianity, “soul” became much more closely identified with “person”. But we can ignore this for now.
Second, Aristotle proposed a theory of successive ensoulments to explain the development of a human being. These stages were vegetative soul, non-rational animal soul and finally rational animal soul. Aristotle argued that the “rational animal soul”, as a “principle of operation”, could not be present if the requisite organs were not also present.
I think Aristotle was wrong about this, even in terms of his own philosophy. The “soul” is not only a formal cause, it is also a final cause, a principle which guides changes to their natural end (in today’s science, “soul” would be closely associated with the DNA but, philosophically, it is not the DNA).
So the “rational animal soul” is present at conception as the “final cause”.
Now why is all this important?
I don’t think the person can be present at the moment of conception unless the “rational animal soul” is also present. Under the theory of successive ensoulment, this is impossible. Ergo, the “person” is not present at the moment of conception.
But if the “person” is not there, then why isn’t direct abortion up to a certain stage of pregnancy permissible?
If you don’t address the question of ensoulment, you weaken the pro-life position.