In times past, the process of human development in the womb was unknown. Great Saints of the past talked of humans going through a “vegitative state” in which they thought we were more like vegetable life than animal (human) life. Since vegatative life does not have a human soul, it was thought that the developing life was not yet human. Believing this, it would be conceivable that abortion could be allowed during that state but absolutely prohibited once a human soul was created (quickening) for the developing person. The time of quickening was determined to be when movement of the baby could be felt by the mother (under the false assumption that the mother would be able to feel any movement so the baby must not have been moving before that time - no movement, no human life, no soul, so abortion would be okay).
Recent advances in medical science has dispelled all of these misconceptions about human development but the important note on the views of the past is that the “acceptability” of abortion was always based on the idea that the child in question was not yet a human because he did not yet have a rational soul created for him by God. Once that soul was present, abortion was absolutely forbidden and that teaching goes all the way back to the first century of the Church.