Some opponents of the death penalty oppose it on the ground that one can never be certain whether the accused is guilty as charged. Some of these people are pro-choice. To them I say: Are you certain that the killing of the unborn is morally permissible? How can you be sure? How can you be sure that the right to life kicks in only at birth and not one minute before? What makes you think that a mere 'change of address,' a mere spatial translation from womb to crib, confers normative personhood and with it the right to life? Or is it being one minute older that confers normative personhood? What is the difference that makes a moral difference — thereby justifying a difference in treatment — between unborn human individuals and infant human individuals?
Suppose you accept the general moral prohibition against homicide. And suppose that you grant that there are legitimate exceptions to the general prohibition including one or more of the following: self-defense, just war, suicide, capital punishment. Are you certain that abortion is a legitimate exception? And if you allow abortion as a legitimate exception, why not also capital punishment?
After all, most of those found guilty of capital crimes actually are guilty and deserving of execution; but none of the unborn are guilty of anything.
My point,then, is that if you demand certainty of guilt before you will allow capital punishment, then you should demand certainty of the moral permissibility of abortion before you allow it. I should add that in many capital cases there is objective certainty of guilt (the miscreant confesses, the evidence is overwhelming, etc.); but no one can legitimately claim to be objectively certain that abortion is morally permissible.