This article by Lara Denis (Canadian Journal of Philosophy December  2007) considers two Kantian arguments against abortion: R.M.Hare's and Harry Gensler's. Click here for the complete article: below is a short extract discussing Hare's and Gensler's views.

Hare’s argument goes like this. Imagine a ‘time-switch into the past’ in which I can speak with my mother when she is considering aborting the pregnancy that would result in my birth. Assume that I am a ‘normally happy person’ and that my existence is valuable to me. Also assume that my mother will not die if she goes through with the pregnancy. Under these circumstances, I would tell her not to have an abortion. Indeed, I would morally advise her that she ought not to have an abortion, for my preference to live and enjoy all life has to offer outweighs my mother’s preference for an abortion. If it is wrong for herto have an abortion in these circumstances, then it would be wrong for anyone in relevantly similar circumstances to have an abortion. Most circumstances in which a woman considers abortion are relevantly similar to that one. So abortion is usually wrong; that is, it is wrong except perhaps in such cases as those in which the woman would die if she completed the pregnancy, or the fetus would not grow into a person
who would ‘be glad to be alive.’

So we should adopt a principle rejecting abortion in general, but allowing for exceptions. Gensler’s approach is similar to Hare’s. Gensler locates the Kantian nature of his approach in certain consistency requirements, the most important of which he sees as derived from requirements of universalizability and prescriptivity, and which he describes as ‘a version of the
golden rule’: ‘If you are consistent and think that it would be all right to do A to X, then you will consent to the idea of someone doing A to youin similar circumstances.’

This principle furnishes the major premise in Gensler’s abortion argument: ‘If you are consistent and think that abortion is normally morally permissible, then you will consent to the idea of your having been aborted in normal circumstances.’ The minor premise is, ‘You do not consent to the idea of your having been aborted in normal circumstances.’ Therefore, Gensler concludes, ‘If you are consistent then you will not think that abortion is normally permissible.’