"Thou shall not murder"?
That human life begins at conception is part of the Ordinary Magisterium, but has not been defined "ex cathedra," as has The Assumption, for example.
Pope Says Pre-implanted Embryo is SacredOf course, no number of papal statements will convince death-minded, caviling theologians.VATICAN CITY --Pope Benedict XVI said Monday that embryos developed for in vitro fertilization deserve the same right to life as fetuses, children and adults -- and that that right extends to embryos even before they are transferred into a woman's womb.
The Vatican has long held that human life begins at conception, but Benedict's comments were significant because he specified that even an embryo in its earliest stages -- when it is just a few cells -- is just as much a human life as an older being.
Well ... it's probably not an exercise of the "extraordinary magisterium," but it's without question infallibly known. JP2 said this (Evangelium Vitae 62):
Given such unanimity in the doctrinal and disciplinary tradition of the Church, Paul VI was able to declare that this tradition is unchanged and unchangeable. Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops-who on various occasions have condemned abortion and who in the aforementioned consultation, albeit dispersed throughout the world, have shown unanimous agreement concerning this doctrine-I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written Word of God, is transmitted by the Church's Tradition and taught by the ordinary and universal Magisterium.