64 As to the "medical and therapeutic indication" to which, using their own words, we have made reference, Venerable Brethren, however much we may pity the mother whose health and even life is gravely imperiled in the performance of the duty allotted to her by nature, nevertheless what could ever be a sufficient reason for excusing in any way the direct murder of the innocent? This is precisely what we are dealing with here. Whether inflicted upon the mother or upon the child, it is against the precept of God and the law of nature: "Thou shalt not kill:" The life of each is equally sacred, and no one has the power, not even the public authority, to destroy it. It is of no use to appeal to the right of taking away life for here it is a question of the innocent, whereas that right has regard only to the guilty; nor is there here question of defense by bloodshed against an unjust aggressor (for who would call an innocent child an unjust aggressor?); again there is not question here of what is called the "law of extreme necessity" which could even extend to the direct killing of the innocent. Upright and skillful doctors strive most praiseworthily to guard and preserve the lives of both mother and child; on the contrary, those show themselves most unworthy of the noble medical profession who encompass the death of one or the other, through a pretense at practicing medicine or through motives of misguided pity.
Source: Casti Connubii (Vatican.va)