In a footnote on pages 243-244 of his book Political Liberalism
(Columbia University Press, 1993), John Rawls offers the
following comments on abortion.

[C]onsider the troubled question of abortion. Suppose first that the
society in question is well-ordered and that we are dealing with the
normal case of mature adult women. It is best to be clear about this
idealized case first; for once we are clear about it, we have a guide
that helps us to think about other cases, which force us to consider
exceptional circumstances. Suppose further that we consider the
question in terms of these three important political values: the due
respect for human life, the ordered reproduction of political society
over time, including the family in some form, and finally the
equality of women as equal citizens. (There are, of course, other
important political values besides these.) Now I believe any
reasonable balance of these three values will give a woman a duly
qualified right to decide whether or not to end her pregnancy
during the first trimester. The reason for this is that at this early
stage of pregnancy the political value of the equality of women is
overriding, and this right is required to give it substance and force.
Other political values, if tallied in, would not, I think affect this
conclusion. A reasonable balance may allow her such a right
beyond this, at least in certain circumstances. However, I do not
discuss the question in general here, as I simply want to illustrate
the point of the text by saying that any comprehensive doctrine that
leads to a balance of political values excluding that duly qualified
right in the first trimester is to that extent unreasonable; and
depending on details of its foundation, it may also be cruel and
oppressive; for example, if it denied the right altogether except in
the case of rape and incest. Thus, assuming that this question is
either a constitutional essential or a matter of basic justice, we
would go against the ideal of public reason if we voted from a
comprehensive doctrine that denied this right. However, a
comprehensive doctrine is not as such unreasonable because it
leads to an unreasonable conclusion in one or even in several
cases. It may still be reasonable most of the time.