There is never actually a case where the fact that the baby is alive is harmful to the mother. If you have evidence against this, please show me. All the medical issues with pregnancies that I have seen or read about have all been about the location of the baby, the size of the baby, or the baby needing more nutrients than the mother can provide (usually her own fault). The two former, I believe, are the more common, especially in arguments.
If the baby is too large that the mother, due to some pre-condition or whatever, is put in danger, the baby is likely big enough already to live outside of the womb (albeit with medical assistance, as it would be premature), and does not require killing the baby. The earliest a child (as far as I know) has survived after having been prematurely born/removed was something like 22-23 weeks old (really early and tiny). The ‘safe-zone’ age is typically around 26 weeks, where doctors are pretty confident of the baby’s survival.
A commonly known problem with the baby’s location is if the child “implants” in one of the fallopian tubes rather than in the womb. In this case, killing the baby isn’t a viable option as you can also really harm the mother. It is much simpler and safer to remove the section of tube where the baby resides. However, the child can (and I believe often does) die during this process, but that is not the intent of the operation. It is not a direct attack on the child, but rather an attempt to treat the condition and the baby happens to be affected. Similarly, if the mother had cancer that required radiation treatments. It is highly likely that the baby could die due to the treatment, or at least severely harmed, but that isn’t the intent. The mother is being treated, and has the unfortunate side effect of harming, or even killing, the child.
So, as I said. There is never a case, to which I am aware, where the actual life of the child is harmful to the mother.