Peter Kreeft from his book Catholic Christianity:
The basic arguments for and against abortion
There are three steps, or premises, to the argument for outlawing abortion:
The first is that one of the most fundamental purposes of law is to protect human rights, especially the first and foundational right, the right to life.
The second is that all human beings have the right to life.
The third is that the already-conceived but not-yet-born children of man beings are human beings.
From these three premises it necessarily follows that the law must protect the right to life of unborn children.
There are only three possible reasons for disagreeing with this conclusion and being "pro-choice" instead of pro-life. One may deny the first, second, or third premise. For if all three are admitted, the pro-life conclusion follows.
Thus there are three different kinds of "pro-choicers":
First, there are those who admit that all persons have a right to life and that unborn children are persons but deny that this right should be protected by law (the first premise). This is a serious legal error.
"The inalienable right to life of every innocent human individual is a constitutive element of a civil society and its legislation.
"'The inalienable rights of the person must be recognized and respected by civil society and the political authority. These human rights depend neither on single individuals nor on parents; nor do they represent a concession made by society and the state; they belong to human nature and are inherent in the person by virtue of the creative act from which the person took his origin...3
"'The moment a positive' " [human] "'law deprives a category of human beings of the protection which civil legislation ought to accord them, the state is denying the equality of all before the law. When the state does not place its power at the service of the rights of each citizen, and in particular of the more vulnerable, the very foundations of a state based on law are undermined' 4" (CCC 2273).
Second, there are those who admit that the law should protect the right to life and that unborn children are human beings but deny that all human beings have the right to life (the second premise). This is a very serious moral error.
It is essentially the philosophy of power, of "might makes right." Those in power—doctors, parents, legislators, adults—decree the right to kill those who lack the power to defend themselves: the smallest, most vulnerable, and most innocent of' all human beings. No good reason can justify this decree; a good end does not justify an intrinsically evil means. If the babies shared the powers of the abortionists and could fight back with scalpels, there would be few abortions.
Third, there are those who admit that the law should protect the right to life and that all humans have that right, but deny that unborn children are humans (the third premise).This is a serious factual and scientific error.
Before Roe v. Wade legalized abortion, all science texts taught the biological truism that the life of any individual of any species begins at conception, when sperm and ovum unite to create a new being with its own complete and unique genetic code, distinct from both father and mother. All growth and development from then on are a matter of degree, a gradual unfolding of what is already there. There is no specific or distinct point in our development when we become human. (What were we before that? Birds?) Only when abortion became legal did the science textbooks change their language and cease teaching this truism—not because of any new science but because of a new politics.
Abortion is not a complex issue. Few moral issues could be clearer. As Mother Teresa said, "If abortion is not wrong, nothing is wrong."