Aquinas created 5 primary precepts that are the basis of Natural Law theory. 3 of those are relevant to this question. The preservation of life, reproduction and the nurture of the young. On all these points’ abortion would not be allowed, although there is the question as to when a foetus becomes a person. The legal age at which a foetus has rights is 24 weeks but some say that life begins at conception and others think it is when the heart starts to beat and the brain shows activity or when the baby is able to survive outside of the womb.
Deciding when the unborn baby becomes a real person is a crucial factor when looking at abortion as natural law is concerned with the good of people in mind. There are various bible quotes that suggest that God sees unborn babies as real humans from the moment of conception that are then automatically covered by the 5 precepts and therefore abortion is wrong. As Natural law wants every human to thrive and strive for eternal happiness cutting a life purposely short goes completely against it.
However, the doctrine of double effect says that
Application: The act of abortion of its very nature is inherently evil, because it is the intentional and direct killing of an innocent human being. This would apply to all abortions, including those in the case of rape and incest (and to those involving human fetal and human embryo research, and human cloning). Therefore it is never morally permissible to undergo an abortion procedure. The principle of double effect as applied to the case of abortion renders abortion procedures morally illicit, since the action by its very nature