Entering into a discussion with pro-abortion advocates always presents a challenge. It’s easy, however, to be taken in by what might be considered two of their stronger arguments: that a woman has a right to do what she wants with her body, and a concern that you are trying to force your religious views on the rest of society. You can significantly weaken their arguments by limiting your discussion to acceptable medical knowledge and some logical reasoning.
While it’s true that a woman does have rights over her body, medical science has shown that at conception the being that is formed is something other than a part of her body. It has a unique DNA, different from either the mother’s or the father’s. And, that new DNA is human DNA to begin with. It doesn’t start out neutral and then suddenly become human. Your pro-abortion friend needs to acknowledge this.
Also, the fact that the new being is small doesn’t mean it can therefore be removed from the chain of normal human development: zygote to embryo to fetus to baby to child to teen to adult to end of life. Since this process of human growth and aging is more seamless than our limited arbitrary designations indicate, no one has a right to prevent a zygote from becoming an embryo, or a fetus a baby, any more than they have a right to prevent a child from becoming a teen, or a teen an adult. All of these are legitimate stages of human development where different sizes and physical/mental abilities are evident. Challenge the pro-abortionist who has incorrectly concluded otherwise.
Using some basic logic and the evidence of medical science regarding the beginning, uniqueness, and development of each human life, you can go a long way to reduce the pro-abortion argument to the gruesome fact that what they really want is a legal right to end a human life merely for the sake of convenience. It is this blatant disregard for the value of human life that the Catholic Church has always taught against and condemned. Make this your stance too, while preparing to defend it if need be using a completely secular approach.