As to basic Buddhist principles, when the mother's life is in danger, yes it's okay, trying to save the mother's life. The intention is to save a life and the fetus is not considered as important as the mother. In that way, one life is sacrificed for another but the intention is to save a life that's already living. So, in a case where there's danger to the mother's life, from a Buddhist perspective, abortion would be okay.
As to rape or incest, according to the Law of Kamma, according to Buddhism, the person who got raped, that was their Kamma to be raped.
Being raped is one issue. But now that they are a woman with a baby growing inside themselves, that's a second issue.
From our perspective the person goes ahead and has the baby, if they don't want it after it's born then they can put it up for adoption. But don't kill the baby simply because the sperm that created it came from someone whom we do not want to be the father, so to speak.
There are differences here. Raping and incest are one issue, in that it is more related to the Kamma of the people involved. However, aborting a baby, no matter where it came from, that's a different issue.
From a Buddhist perspective, go ahead and have the baby. But of course if the mother doesn't want to keep the baby because it came about through rape or incest, then it's quite okay to put the baby up for adoption.
The key aspect here, is to really look at each action individually. With rape, with incest, we want to stop it. We want to stop it as quickly as we can. If we could, we want to try to stop it before it happens. So if we know a person who rapes, then we would like to take that person, put them in jail or in an institution to try to correct their wrong thoughts and their wrong actions. We would separate them from society.
But abortion is another issue. When a mother has a baby growing inside them, that's a completely different issue.