Hi there! So a couple things to address this question. First, I want to contextualize this argument because I have often heard health challenges for the mother used as an argument to justify all abortions, including those in perfectly normal and healthy pregnancies, and I do not believe this is a legitimate argument to justify abortion.
First of all, late-term abortions (when the baby has the chance to survive outside of the womb) or partial-birth abortions are never needed to save the life of a mother. In dangerous situations, babies are delivered, often via c-section, such as in the case of Placenta Previa which threatens both the mother and the baby. So this argument has to be restricted to pregnancies very early on when the baby will not survive outside of the womb, and health challenges cannot be used to justify late-term or partial-birth abortions.
In the case of very early abortions where the baby will not survive if delivered, life threatening situations in the case of pregnancy are very rare. Even Planned Parenthood director said, "Today it is possible for almost any patient to be brought through pregnancy alive, unless she suffers from a fatal illness such as cancer or leukemia, and, if so, abortion would be unlikely to prolong, much less save, life.” So it is an extremely small percentage of lives that are threatened by pregnancy and this small percentage should not be applied to all abortions and be used to justify it.
So just to put this in context, health complications cannot justify abortions where the child has to chance to survive outside the womb and are rarely an argument for early pregnancies. But let's say we are in the very rare situation where the mother's life (not health but life) is in danger. First of all, a doctor cannot say with certainty that the mother will die if the pregnancy continues. There are cases where the risk is high that she will not survive the pregnancy, but doctors cannot predict the outcome with certainty, and in fact there are many incidents when a doctor has said a mother may die if her pregnancy continues, yet she chooses to carry her baby and both she and the baby survive the high-risk pregnancy just fine. So I don't think an abortion is justified if the mother "might" die because if there is the chance both she and the baby will live, the doctors should do all they can to keep both the mother and the baby alive, rather than killing the child. I don't believe a possible death is justification for a certain death. And because there has not been a situation where a mother is guaranteed to die if the pregnancy continues, I believe that instead of ending the life of the baby, the solution should be to fight for the lives of both the mother and baby. Situations where lives are lost during pregnancy is terrible and heartbreaking, so thankfully we live at a time when medical technology can do so much to protect the lives of mothers and babies involved in high-risk pregnancies.