The discussion of what to do with frozen “spare” embryos is a complex debate. The following text is an interesting read. It is written by Mauro Cozzoli and is an extract from The Human Embryo: Ethical and Normative Aspects. The article is published by the Pontifical Academy for Life in The Identity and Status of the Human Embryo pp. 292-296.
The techniques of artificial fecundation today raise ethical problems with regard to the existence in life and to the destiny of embryos outside the maternal uterus.
This is an anomalous condition, artificially produced. It would be legitimate only for a therapeutic benefit for the embryo, necessary and not otherwise attainable, obviously within the limits of respect for the moral truth of human procreation.
In fact, however, today there are extrauterine embryos for reasons that are extraneous to them: to satisfy desires of maternity, to utilize them for research and experimentation, to make use of them as “biological material” for transplant surgery and pharmaceutical production. These are unacceptable and deplorable reasons for the diversion and emptying of value of the life of the human embryo, in which he is willed not for himself but for others and for other ends. It becomes still more deplorable when the life of the embryo is not guaranteed and is violated.
The extrauterine condition of human embryonic lives raises particular ethical problems with regard to cryopreservation, selection, commerce, and the destination of surplus embryos.
Cryopreservation is a suspension of embryonic development. By means of being frozen in liquid nitrogen, generated human individuals are conserved at very low temperatures in biological immobility.
This constitutes an abusive interference with the life cycle. A human life, once originated, must have its natural course, which no one can interrupt. Temporal and historical continuity is an intrinsic good and a right, by reason of which also an individual comprehends himself. Age is more than a temporal connotation: it is a coordinate of personal life that determines it in the conditions of time and space. To alter it, provoking a vacuum of time in the existence of a person, is a caprice that upsets sense and conscience.
Arresting the vital cycle of a human embryo is an expression of the “will to power”, by which one person decides the fate of another, who happens to be weak and undefended. His dignity is reduced to the value of something to be used, subject also to expiration, since the physical integrity and even the vitality of a frozen embryo cannot be guaranteed, in direct proportion to the time and modality of freezing. Thus in addition to an excess of power, there is joined the violence by which these “expired” and “unserviceable” lives are disposed of.
Selection of the embryos to be implanted in the uterus is a strong temptation, when one disposes of them according to one’s own will. The embryo is not appreciated for his intrinsic value but for the result to be gained from him. His value is related to the effect that the technician seeks to accomplish: to guarantee a pregnancy and a life that can be qualitatively appreciated by the client. This leads him to check the qualifications and conditions of life of the embryos before implantation, in order to “discard” those that do not correspond to the prerequisites. However, selection is also practiced in the uterus, when the surviving embryos which have been implanted are too numerous or do not possess the expected qualities. In this case one proceeds to eliminate those that are not accepted.
In this way the embryo becomes an “object” of desire. He is not treated as a good in himself and therefore as a gratuitous “gift” to be welcomed with an unconditional and grateful freedom. Maternity and paternity here are not a sacred trust and an offering to life.  They become a calculation and a purchase order, the anxious seeking for a life of quality. Thus the way is paved for a mercantile logic of supply and demand, supported by a market of human reproduction, in which not only the gametes but also embryos become “merchandise”. It matters little if the costs are carried by the clients or by the social community, on the presumption that a “child at any cost” and a “life of quality” constitutes a civil right and an obligation of the common good.
The selection of embryos is a practice connected to the techniques of human reproduction. With them the logic of “production”, which governs these techniques, is exasperated, as we have said, into a mentality of “domination” over life. This progression follows an inherent logic. When the logical and axiological difference between procreation as “donation” and procreation as “production” of life is no longer perceived, procreation becomes something that can be constructed and treated as an option like any product of ingenuity and technology.
Commerce of embryos is a consequence of their extrapolation from the maternal procreative process and of their being valued and treated as an object. A product of the laboratory, embryonic life risks becoming a useful good to satisfy needs, available upon request. The mercantile value that is acquired in this way is a further expression of that expropriation of dignity to which the embryo is exposed today and which is further aggravated by any commercial practice or consideration.
The value of a human life is inestimable and gratuitous. Therefore, any commercial production of human embryos, and the commercial use of embryos not directly produced with this intention, but which are nevertheless available, is deplorable.
In this commercialization, the “economy” of self-donating and welcoming love governing human procreation is completely lost, for the benefit of that other economy which prevails in the world of objects, in which production, domination and commercialization go together. Not to perceive and respect this diversity leads one to consider and treat embryonic life – that which is most exposed and undefended – as a good on the order of an object, to be used at the will of the possessor.
The production of embryos in vitro, destined to satisfy the desire for maternity of women and sterile couples, is raising grave questions about the destiny of supernumerary or spare embryos. They are embryonic lives in cryopreservation which the parents no longer require. Their existence is brought about in order to make up for the high rate of failure of the techniques of fecundation in vitro and subsequent transferal to the uterus. For every woman or couple who seeks to make use of these techniques, a certain number of embryos are produced and frozen in order to constitute a reserve from which they can draw until their desire for a child is satisfied. In the case in which the clients decide that they no longer which a child or they do not wish for any more, the remaining embryos become supernumerary and surplus.
Troubling ethical questions arise with regard to their destiny: What should be done with them? To whom do they belong? Who decides for them? The problem is grave both because of the good that is in question – human life – and because of the incalculable number of surplus embryos in the many centers of human fertilization.
Clearly to be rejected is any destination incompatible with the principle that human life cannot be made use of as an object, such as allowing these embryos to be used for experimentation or for the production of tissues and fetal organs. However, to prolong the freezing puts off and does not resolve the problem, but rather aggravates it both from the biological and moral perspectives. Two solutions remain: implantation in the uterus of women volunteers, and their elimination. Both raise grave ethical problems. The voluntary and direct taking of an innocent human life is immoral. A surrogate pregnancy is also an illicit path for the gestation of a life. In this case, however, it is not intended to satisfy desires but to save a life. Therefore, more than a surrogate pregnancy, it is an adoptive pregnancy.
However, this does not constitute a solution to the problem. Above all because it is an extraordinary means, to which no one can be obligated and which cannot become ordinary. Besides becoming an undifferentiated means of access to maternity (for any woman or couple, in any condition), it would favor recourse to maternity separated from matrimony and sexuality. It is also a disproportionate means to deal with and resolve a problem of enormous dimensions and in progressive expansion. One fails to glimpse either the will to resolve this problem or even the beginnings of projected solutions. Furthermore, in the absence of a willingness and an agreement to cease the practice of the production and freezing of embryos, the systematic recourse to adoptive pregnancy can have the effect of legitimizing and motivating that practice.
The blind alley and the dark tunnel evoked by these contradictions and this impasse, show us the original dilemma and fundamental ethical error which these practices involve. The thought that to save a life one must violate other values, and thus add injustice to injustice, is alarming and disheartening. If one fails to see a way out that can be humanly approve, it is because the road that has been undertaken with artificial procreation is blind and dark, so that the only solution is radical: to turn back and refuse to continue on it.
In the meantime the hundreds of thousands of surplus embryos constitute a real problem that calls for a concrete and inescapable solution. Normally, this will consist in allowing the final act of life which is death. Causing death is different from permitting it. In the first case it is procured with an action that takes away life, or by an omission of assistance and support. In the second case one permits its decease, by removing the life from unnatural and unworthy treatments and environment, while continuing to respect it in its decease. Ethics obliges recourse to ordinary and proportionate means of maintaining life, not to those that are extraordinary and disproportionate.  Such would be the cryopreservatation of an embryonic life without an acceptable prospect of gestation and the systematic and binding recourse to adoptive pregnancy.
To allow the death of a life in its initial stages nevertheless represents a check, which provokes a sense of impotence and delusion, which is that much more disturbing the thicker and more impenetrable is the curtain of complicity and silence surrounding procreative techniques and their diffusion. We are forced to move within these contradictions and this dismay: a symptom of the impassability of the path that has been undertaken to satisfy the desire for a child. The question is once again brought back to the beginning, in which alone it can find a solution. The instruction Donum vitae had already warned us of this, by radically calling into question the production of “embryos which are not transferred into the body of the mother…, exposed to an absurd fate, with no possibility of their being offered safe means of survival which can be licitly pursued”. 
 Cf. G. Marcel, Homo viator, Aubier, Paris 1945, pp. 92-170.
 Cf. EV n. 65; Congregation for the Doctrine of the Faith, Declaration on Euthanasia “Iura et bona”, May 5, 1980, part IV.
 Cf. DV part I, n.5.