It is true that the Church has never taught that there is a certain moment when the rational soul in infused into the body of the zygote or fetus so that it is a fully human person. Even so, it is clear that abortion is immoral in any case. This is true for two reasons: first because if there were a doubt about the full humanity of the embryo (and, scientifically, there is not any at all), then it still remains true that in case of such doubt one may not proceed to kill the embryo.
For example, if I am hunting and something stirs in the bushes, am I allowed to shoot if there is a reasonable expectation that the noise in the bushes is a human being and not a deer? Of course not—and so in the case where it is not yet certain that there is a human person present, it is still gravely immoral to kill that life, since there is also a reasonable expectation that it is a human person.
And then there is a second reason: there is the basic precept of the natural law that parents seek the conservation of the life of their offspring. A mother conserves the life which is in her and depends on her; she does not destroy it. This is not some kind of imposition by a patriarchal society but a universal norm of all kinds of beings, and most of all human beings.
Here is a passage from the Congregation for the Doctrine of the Faith that pertains to the point we are making:
Certainly no experimental datum can be in itself sufficient to bring us to the recognition of a spiritual soul; nevertheless, the conclusions of science regarding the human embryo provide a valuable indication for discerning by the use of reason a personal presence at the moment of this first appearance of a human life: how could a human individual not be a human person? The Magisterium has not expressly committed itself to an affirmation of a philosophical nature, but it constantly reaffirms the moral condemnation of any kind of procured abortion. This teaching has not been changed and is unchangeable.