Many unborn children are loved as if they bear the kind of intrinsic value that would make killing them wrong. If those unborn children do not have such value, then those who love them are loving irrationally (or are otherwise mistaken or confused); but it is implausible to say that pregnant women are loving irrationally. Therefore (via modus tollens), those unborn children really do have intrinsic value that precludes killing them, and since there are no morally relevant differences between unborn children who are loved and those who are not (the opinion of others cannot determine their moral status), it follows that all unborn children are valuable and ought not be killed. So abortion is wrong.
The decisive premise, it seems to me, is that mothers are not mistaken when they love their unborn offspring. This intuition is so strong that I doubt many pro-choice advocates would deny it, and many have loved their own unborn children and surely thought themselves reasonable in doing so. Pruss writes:
It seems not only a sociologically natural kind of love, but a perfectly rational love. It would be implausible to suppose that the loving mother is in the throes of some conceptual confusion or is ignorant of some relevant fact. But if the love is perfectly rational and not ignorant, then the object of the love has at least the kind of value that it is loved as if it had. Therefore, plausibly, the fetus has the kind of value which justifies the mother's love. But the amount of value which the mother in her love predicates of the child is such as would make killing the child prima facie wrong. Hence, abortion is prima facie wrong.Pruss goes on to answer a number of objections to this argument. The bottom line is that the apparent reasonableness of maternal love for unborn children is significant evidence that abortion is morally impermissible. To say that abortion is permissible, on the other hand, one would have to hold that perhaps a majority of pregnant women (by loving their unborn children as individuals who really matter) are behaving irrationally or out of ignorance or confusion!