he very rare cases of pregnancy that pose a real and immediate threat to the mother's life including uterine cancer and ectopic pregnancies are a source of great confusion, especially among Catholics.
It is absolutely true that the Catholic Church bans abortion to save the life of the mother.
However (and this is an extremely important point) the mother's life may be saved by a surgical procedure that does not directly attack the unborn baby's life.
The most common dysfunctions that may set a mother's life against that of her unborn child's are the ectopic pregnancy, carcinoma of the uterine cervix, and cancer of the ovary. Occasionally, cancer of the vulva or vagina may indicate surgical intervention.
In such cases, under the principle of the "double effect,"
attending physicians must do everything in their power to save both the mother and the child. If the physicians decide that, in the case of an ectopic pregnancy, the mother's life can only be saved by the removal of the Fallopian tube (and with it, the unborn baby), or by removal of some other tissue essential for the preborn baby's life, the baby will of course die. But this would not be categorized as an abortion. This is all the difference between deliberate murder (abortion) and unintentional natural death.
The principle of the "double effect" also applies to sexual sterilization. If a woman must have a hysterectomy to remove a dangerously cancerous uterus, this will result in her sterilization, but is not a sinful act. However, if the purpose of the operation is not to heal or safeguard health, but to directly sterilize, then that act is intrinsically evil and is always a mortal sin.
http://www.ewtn.com/library/PROLENC/ENCYC043.HTM