I still think that they can be grey cases even when one holds that the only justification of abortion is to save the mother's life. It's possible that the mother has a medical problem which means that there is a probability that she will die from continuing the pregnancy but it is not certain. In that case there's a judgement call to be made about how high a probability is worth risking. Pregnancies always involve some risk, how much is too much? There are also judgement calls to be made in cases where the mother needs life saving treatment which she can't have whilst pregnant and the baby is delivered prematurely so that she can have the treatment earlier. There can be a trade off between the increase in the baby's chances from waiting against the decrease in the mother's chances. I don't know how those sorts of judgement calls work in Catholicism.