It is the moral and legal obligation of the doctor to attempt to treat both patients (mother and child) to the best of his ability. The child should be kept in the mother as long as possible to enable it to get stronger and more likely to survive, and should be delivered when necessary for the mother despite whatever risks it may face, including possible death from prematurity.
In this case, the point of the treatment is not the direct destruction of the child, but an attempt to save both mother and child, even if the chances for the child are slim.
Some other maternal health problems that may be indications for abortion: cardiac disease with cardiac decompensation, and certain malignancies. I'm curious - where do you draw the line?
All direct abortion (surgical operations performed with the first intended purpose of killing the child) is immoral and should be illegal. For example, it is not immoral for a pregnant woman with cancer to take medicines to treat the cancer, even if it may prove harmful to the child. The intent here is not the harm of the child, although that might be an undesired side-effect but the cure of the woman. Many women have praiseworthily taken it upon themselves to delay their treatment in order to give their child a chance to be born prematurely with a chance to live - in this day and age, this sort of delay would generally average out no more than 2-3 months.
I've become especially curious about this after an incident with a nurse in an ER I worked at. She had an ectopic pregnancy. Her husband, a minister, encouraged her not to have treatment. She was fortunate - she passed out in the ER and had immediate surgery and survived. I had to question the morality of a husband who would encourage his wife to do something very likely to lead to her death.
Well, that is just plain dumb. An ectopic pregancy requires removal of the diseased portion of the fallopian tube. The death of the child is an unfortunate side-effect, and not the directly willed result of the operation. The operation is not an abortion procedure aimed at destroying the child, but a surgery aimed at removing a diseased part of the woman.
Because of an amazing lack of interest for the most part by medical science, few attempts have been made at reimplantation of the child during this procedure, although it has been done successfully on occasion. One would think in a world obsessed with in vitro fertilization and the like that this sort of reimplantation would be made routine by modern medical technology and knowledge.