The answer is simple: knowledge about procreation, conception and when life begins was very limited in the Middle Ages. Bold theories such as those that "ensoulment" took place 40 days after conception disguised the lack of knowledge. It was science that showed the faults in the bold theories advocated by the Church. For example, Aquinas's view, that up to a certain point the child was part of the mother (pars et portion viscerum matris) and could therefore be destroyed, was proved wrong by the 17th century, when William Harvey discovered that after only three weeks the child had blood circulation of its own.
Today, we know that the ovum contains the complete genetic programme for an individual. Those who are still, at the dawn of the 21st century, advocating abortion betray any values conveyed by the Enlightenment, and take the view of the Middle Ages. The difference is that they cannot claim the innocence of the ignorant.
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