As a Catholic with mostly Protestant, agnostic, or atheistic friends, Eric Scheske gets a lot of, sometimes pointed, questions about when it's permissible for a woman to get an abortion. Eric always answers, "Never," to which he gets a lot of other, even pointier, questions. Most particularly: "What if the mother will die if she carries the baby to term?" Eric then tries to explain the principle of double effect: It is morally acceptable to undergo treatment for a serious medical condition even if the therapy is potentially harmful, even lethal, to an unborn child. The intent, however, cannot be to kill the child. "It's all in the intent," you might say.
Anyway, here's an article about double effect and the Catholics's theological right to ignore it. If a mother wants to forego legitimate treatment because it might harm or kill the fetus, she may. Indeed, the Church may even consider her actions heroic. Not for the weak of heart, stomach, or faith: LINK