From an L.A.Times editorial, "Stem Cell Hypocrisy":
[E]ncouraging the donation of frozen embryos to prospective parents, even under the most optimistic scenario, would put only a small dent in the supply. According to a 2003 study, there are almost half a million frozen human embryos in storage in the United States. The vast majority of them — 87% — were frozen in case the parents might need them, but the vast majority of that vast majority will never be needed or used. An embryo-adoption drive wouldn't save the embryos that die in other stages of the process. And ironically, the recipients of donated fertilized eggs also generally have several implanted in the hope that one will survive. In effect, donation results in the deaths of embryos that would otherwise stay frozen.
Half a million!
Bottom line: Those who oppose embryonic stem cell research on moral grounds must either oppose in vitro fertilization, or stand convicted of hypocrisy. If frozen embryos can be discarded and destroyed, then there can be no objection to donating their cells for research, the way you might donate a dying relative's organs for transplant. If frozen embryos cannot be discarded and destroyed, then they should never be created, even if it means that some couples must reconcile themselves to infertility and adoption.
I would like to propose a third alternative.
Devout Catholics will disagree, but I think that life proper can accurately be said to begin with successful implantation. Many naturally conceived embryos (some estimates run as high as 50%, but we'll never know) do not succeed in implanting in the womb, either because of some factor in the embryo or because the woman's body is not receptive at the time. It may be no coincidence that embryos can easily be grown in a Petri dish and then frozen at the 8-cell stage, and with more sophisticated techniques, at the blastocyst (hollow-ball) stage, at which the embryo is ready to implant -- but no further.
The fact that their development can be suspended and then restarted might encourage us to think of these embryos as "people seeds." Each is the seed of a unique individual, but only at the moment when that seed is accepted by a woman's womb, sheds the "seed coat" of the zona pellucida, takes "root" and begins to grow, does that individual's potential life become actual, acquire "a local habitation and a name."
This would be, to me, both a metaphorical and a literal expression of the fact that there is no human life without relationship. We would not have a language, a name, the ability to survive, or even an existence without one another -- without at least one other, a mother, and all the relationships that in turn support her.
It would also be an argument for an emergency contraceptive like "Plan B" as the last threshold short of abortion. This will not move anyone who believes it should be up to God, never us, whether an act of sex creates a life or not. But it seems to me that a woman's psychological or relational or economic unreadiness to be a mother might be as legitimate -- if not as "innocent" -- as any involuntary biochemical reason for her uterus to be unreceptive.
I understand and even partially agree with the arguments that life or God often knows better than we do, and that things we're unready for can turn out to be -- especially given an attitude of principled surrender -- some of the best things that ever happened to us. But they can also be some of the worst. For every story of an unplanned child becoming a blessing, there is one of awful suffering all around. The argument that we may be turning away the genius who would find a cure for cancer can always be answered by the argument that we're just as likely (i.e., not very) to be turning away a future mass murderer. Abortion is violent because it rips out a life that's already on the way, breaks a bond that has already been struck. "Plan B" is less like a two-edged sword than a "No Vacancy" sign.
If we thought of very early embryos as "people seeds," it might be less problematic to accept that, whether in the fallopian tube or in a fertility clinic, not every seed gets planted.
UPDATE: William Saletan in Slate writes that sure enough, pro-lifers are gearing up to restrict in-vitro fertilization, so that a couple will be allowed to create only one or a few embryos at a time.
I have serious doubts about the wisdom of IVF. But I'm also frightened by the absolutism of people who proclaim every zygote's right to life, when God or Nature itself recognizes no such thing. The real question, it seems to me, is not whether every "people seed" ought in principle to be planted, but whether human will is or is not one more legitimate factor in determining which, or when. (That is, I'm more interested in Plan B than in IVF.) The notion that hormonal fluctuations or imbalances are from God, while family planning is not, is at the very least an interesting one. The Catholic assumption, I think, is that as long as these matters are entirely beyond our control, they are heaven-sent -- whether what Heaven sends is infertility, Down's syndrome, or an eleventh child.