Such was an issue that quickly distinguished the early Christian community from its pagan neighbors. Thomas Aquinas is mentioned by name as holding that it occurred 40 days after conception for males and 80 days after for females. However, no mention is made that St. Thomas also inherited a faulty view of biology, believing that the whole person in potency was locked in the man's seed. Further, St. Thomas still considered the destruction of this person in potency to be a serious moral wrong. He did not accept abortion as a legitimate response to pregnancy.
Modern biology supports the view of immediate hominization.
In any case, since we cannot empirically observe ensoulment, all believers must logically opt for the alternative that causes least harm, giving the unborn the benefit of the doubt. Things cannot be preserved or cleansed from sin, only persons. Thus, one could argue that like Mary, all are persons at the very first moment of conception. All human persons, from the womb to the tomb, are deemed as possessing a precious and irreplaceable dignity. All human life is incommensurate and beyond any utilitarian value.