Double Effect

The Principle of Double Effect is possibly one of the worst tongue-twisters in the world of Catholicism. I can't convey how many times I've heard the sentiment that the Church "permits abortion in some cases" when the person speaking actually (or...probably...I can't say for certain) was talking of the principle of double effect (i.e the hypothetical case of an ectopic pregnancy [tubal]). So, here is the Principle in a nutshell.

The act committed must not be an evil in itself; it ought to be good or at least morally indifferent. The intended good must not result from the evil, and the evil cannot be intended, only tolerated. Lastly, there must be a legitimate reason for taking action in the first place.

Therefore, for a mother with, say, cancer, it is perfectly moral to accept treatment even if the treatment will, in the process of curing the cancer, cause complications to the pregnancy and very likely kill the child. It is moral because the goal is not to kill the child, but to defeat the cancer and save the woman's life--the child's death is not intended, only tolerated.

Voila.

See also: http://www.lifesitenews.com/ldn/2010/may/10051712.html