I believe the 3 month mark is a good way to go. Beyond that, I'd say only if there are grave dangers to the mom and/or baby to continue the pregnancy. Medical, physical dangers that is.
Late term abortions (after the 3rd month) I believe should be extremely rare and should be decided by the mom first, and if she says yes, by a team of doctors second. They are to decide, by a guideline, if the life of the mom is really in danger and if the life of the baby really would be better of ended instead of lived.
I must admit that that last one is a very difficult line for me personally. I wouldn't kill a born baby because it is ill, but I would a fetus?