Quote from the Catholic Encyclopedia article on Creationism:
The Catholic Church teaches that souls are created at their infusion into the body" (Sent. II, d. xviii); while St. Thomas is more emphatic: “It is heretical to say that the intellectual soul is transmitted by process of generation” (I, Q. cxviii, a. 2). For the rest, the following citation from the
Angelic Doctor sums up the diverse opinions: “Regarding this question various opinions were expressed in antiquity. Some held that the soul of a child is produced by the soul of the parent just as the body is generated by the parent-body. Others maintained that all souls are created apart, moreover that they are united with their respective bodies, either by their own volition or by the command and action of
God. Others again, declared that the soul in the moment of its creation is infused into the body. Though for a time these several views were upheld, and though it was doubtful which came nearest the truth (as appears from Augustine’s commentary on Gen., x, and from his books on the origin of the soul),** the Church subsequently condemned the first two and approved the third” (De Potentiâ, Q. iii, a. 9). **