Aquinas’ Natural Law has no clear directive on abortion, the tradition of the Roman Catholic Church has been to reject all forms of abortion.
The key debate is to at what point potential human life begins to make abortion an ethical injustice.
Since the magisterium teaches that life begins with conception, it is clear that abortion is contrary to Natural Law because it directly breaks the first precept.
Assisted fertalisation:
assisted fertalisation through selection of embryos is also condemned by the magisterium since fertilisation and conception are seen as the same
it refers to this practice as ‘selective abortion’.
However, it does not necessarily follow that all Roman Catholics reject abortion since there are different arguments presented within Roman Catholicism.
Exceptional circumstances:
Others would accept the Roman Catholic position in general but also argue that there are exceptional circumstances such as rape and incest.
Even in Christian theology, Aquinas appears to recognise the humanness of a foetus at ensoulment. (when a human gains a soul)
Choice:
Others, like Kainz argue that the issue of abortion is also about the Primary Precept involving a woman’s right to choose whether or not to reproduce.
There are some Roman Catholic pro-choice groups (e.g. Catholics for Choice).