This post draws largely from Peter Kreeft's podcast entitled Pro-Life Philosophy. He is a Catholic Christian apologist and professor at Boston College. Below is an abridged version of his pro-life argument based on the format of Blaise Pascal's argument for the existence of God.
Pascal's Wager:
1) God exists and I believe in Him,
2) God exists and I don't believe in Him
3) God doesn't exist and I believe that he does,
4) God doesn't exist and I don't believe that He does.
In 2/4 cases the person is right, but if the person is a skeptic, he/she remains undecided. However, one is embarked on the journey of life and so they have to choose.
1) God exists and you believe in him=Heaven/infinite happiness,
2) God exists and you don't believe in him=no happiness,
3) God doesn't exist but you believe in him anyway="john lennon is right, as is vladimir lennon"
4) God doesn't exist and you don't believe he does-you are right, but gain nothing in terms of happiness, no payoff-you're dead.
So the only combo for winning anything is God exists and you believe in him. This is not necessarily an air tight argument for God's existence and Pascal is not the focus of this post. However, Kreeft uses the same format of Pascal's wager and asks the abortion skeptic to consider his quadrilemma:
What is abortion in each of these four cases?
1) If you know that the fetus is a person and you kill him/her=murder
2) If you know that the fetus is not a person and you kill it? It's okay, it is excising cells.
3) Don't know that the fetus is a person, but in fact it is, and you kill it anyways=manslaughter
4) Don't know that the fetus is a person, and it happens that it is not a person and you abort=lucky but morally reprehensible
Kreeft likens the last two choices to these scenarios: If you are driving on the highway at night and you see a box and there appears to be something in it, but you can't quite make out what it is, would you drive over it anyways and hope that there was no one in there? Or if you and your partner are hunting in the woods but have become separated and something moves in the bushes 20 yards from you, do you shoot at it and hope that it was not your partner?
"Pro-lifers get accused of claiming to know when life begins however can a pro-choicer prove that it doesn't begin at conception? The skeptical argument works in favor of anti-abortion rather than the reverse. If we can agree to be skeptical, this is the strongest possible argument for not having an abortion."