According to Donum Vitae: Human embryos obtained in vitro are human beings and subjects with rights: their dignity and right to life must be respected from the first moment of their existence. It is immoral to produce human embryos destined to be exploited as disposable “biological material”. In the usual practice of in vitro fertilization, not all of the embryos are transferred to the woman’s body; some are destroyed. Just as the Church condemns induced abortion, so she also forbids acts against the life of these human beings.
It is a duty to condemn the particular gravity of the voluntary destruction of human embryos obtained ‘in vitro’ for the sole purpose of research, either by means of artificial insemination of by means of “twin fission”.
By acting in this way the researcher usurps the place of God; and, even though he may be unaware of this, he sets himself up as the master of the destiny of others inasmuch as he arbitrarily chooses whom he will allow to live and whom he will send to death and kills defenseless human beings.
Methods of observation or experimentation which damage or impose grave and disproportionate risks upon embryos obtained in vitro are morally illicit for the same reasons. Every human being is to be respected for himself, and cannot be reduced in worth to a pure and simple instrument for the advantage of others. It is therefore not in conformity with the moral law deliberately to expose to death human embryos obtained ‘in vitro’. In consequence of the fact that they have been produced in vitro, those embryos which art not transferred into the body of the mother and are called “spare” are exposed to an absurd fate, with no possibility of their being offered safe means of survival which can be licitly pursued.
Congregation for the Doctrine of the Faith, Instruction on Respect For Human Life In Its Origin And On The Dignity Pf Procreation Replies To Certain Questions Of The Day, #5.