No matter how adversaries in the abortion debate choose to define their respective points of view, the primary point of contention in the issue ultimately reduces to one simple question...when does life begin? This reality has become particularly evident as the question has now begun to dominate discussions over the potentials of stem cell research and calls for legislation related to fetal homicide.

As a developmental biologist, I might contend that the determination of when human life begins is properly the responsibility of those of us whose academic credentials and research experience have equipped us to understand the details of human development, and to sort through them objectively. In principle, this is a valid contention. In practice, however, such objectivity–even among credible scientists–is too often corrupted by ethical, religious and political biases. Recognizing these disagreements, Justice Blackmun, recording the opinion of the majority of the U.S. Supreme Court in its decision in Roe v. Wade, wrote: "We need not resolve the difficult question of when life begins. When those trained in the respective disciplines of medicine, philosophy, and theology are unable to arrive at any consensus, the judiciary, at this point in the development of man's knowledge, is not in a position to speculate as to the answer."

What Justice Blackmun failed to acknowledge in his opinion was that the failure of consensus is not because "man's knowledge" has not advanced sufficiently to make a consensus possible, but rather because the facts issuing from that knowledge continue to be obscured by bias. Also absent from his discussion was any acknowledgement that the common origin of that bias is the simple reality that it is easier to play around with the definition of when life begins than it is to enter an ethical minefield requiring decisions about when it is acceptable to terminate it.

Nonetheless, any halfway competent developmental biologist can tell you exactly when human life begins–at least from the biological perspective. To make such a judgement, we are simply required to remain true to the facts and to satisfy two particularly severe critics–simple logic and plain old common sense. Of course, our definition will still not necessarily satisfy those for whom matters of faith are accepted as transcendent to physical reality. However, as it turns out, if you take the beliefs that are typically used to make a theological determination of the beginning of life and then submit those beliefs to the scrutiny of the same two critics, you come up with exactly the same definition as that determined by the biologist.

So, our two critics ask, what are your requirements for a defensible biological definition of when life begins? It will be at that point in time, we answer, after which there are no discernable discontinuities in the developmental process–when everything in the process is totally predictable and nothing further occurs as a matter of chance.

The process leading to the development of a new human life begins when an egg, released from the ovary of the female, is encountered and fertilized by a male sperm. This encounter–and the initial cell divisions in the resulting embryo–occurs in the oviduct of the female. As this process is underway, the embryo travels down the oviduct to the uterus, a trip that requires about a week. By the time it reaches the uterus, the embryo, now called a blastocyst, has become essentially a ball of undifferentiated cells. If development is to continue, the blastocyst must now implant into the wall of the uterus. If implantation does not occur, development will not continue and the embryo will be discarded.

Just how efficient, our two critics ask, is this process of implantation? How frequently do embryos fail to implant? The answer is that implantation is a very chancy process. Embryos fail to implant almost half the time. Thus, because implantation occurs essentially as a matter of chance, the critics require us to conclude that life cannot begin before implantation.

Then, what do the facts tell us about subsequent development–those events following implantation, through gestation and leading to the birth of a child? What they tell us is that there are no further discontinuities. Various points during gestation have been used in the past to identify the time when life begins, but none of them really identifies a discontinuity in the process. In the 19th century, "quickening," the first occurrence of detectable movement by the fetus, was used as one such indicator. More recently, "viability" has become popular. Viability is defined as that point when the fetus has a reasonable probability of survival if it is prematurely removed from the womb. Some people continue to use live birth as the time when life should be considered to begin. But, the fact is, birth itself is nothing more than a trivial change in life support for the developing individual. Biological development proceeds as a totally predictable, continuous process from implantation into adulthood.

Clearly, the facts and the demands of logic and common sense compel us to define the beginning of life biologically as that time when an embryo successfully implants into the wall of the uterus. However, as logical and obedient to common sense as this conclusion may be, it will probably not satisfy those whose religious beliefs compel them to conclude that life begins at conception, the point in time when a sperm and egg unite. So, we will submit their conclusion to the scrutiny of our two critics and see where it leads us.

What are the requirements, the critics ask, for a defensible theological definition of when life begins? The answer most often given identifies that point in time when the new life becomes invested with the "Image of God," the characteristic that makes humans uniquely special in the eyes of the Creator and set apart from the rest of creation. What is the evidence from documents of faith that support the contention that God sets apart the new individual at the point of conception? The answer is that there is no direct evidence. Holy writ does not tell us anything about when life begins. (There is, however, some indirect evidence from scripture concerning when life does NOT begin...and that is at conception--see www.ocomm.net/comment/hebrews.pdf.) So, for most people who believe life begins at conception, it is simply accepted as a matter of faith—it becomes, essentially, an ideology.

OK, the critics continue, let us assume that a new individual becomes the Image of God at the instant of conception. Then, if this developing embryo has been set aside as special and worthy of bearing God's image, how do you account for the fact that almost half of them are destined to pass on through the female's reproductive tract to be discarded? That seems a horrible waste of life–far greater than that which one would expect from any subsequent abortions.

There is, our critics suggest, a more rational alternative to conception, which satisfies all the stated requirements for a theological definition and avoids the logical pitfalls we just encountered. That alternative is implantation, which as a matter of simple logic, is a much more reasonable point in development to identify as the moment in time when the individual becomes the Image of God. First of all, as we explained in arriving at the logic for a biological definition, human development is consistently predictable and continuous only after implantation. Second, depending on how you perceive the manifestations of the Image of God in human character, implantation could be seen as a far more logical opportunity for that quality to be acquired by a new life. At the instant of implantation, a bond–both physical and emotional–is formed between the embryo and the mother, a bond that clearly transcends all other kinds of human relationships. What better time for the new life to be invested with the Image of God than when it first associates intimately with that current bearer of His Image whom He has given responsibility for nurturing it.

The difference in time between conception and implantation is only about a week. That may not seem like much time to fuss about, but the implications for medical research are monumental. For example, if life begins at implantation, then all the commotion about human cloning will have to take a different track. It may still be deemed an ethically bad idea, but the suggestions of some that cloning is the creation of life will no longer wash. Of greater importance are the implications for stem-cell research. Embryonic stem cells could be a significant source of research material for those investigators who envision being able to use them for finding new ways of treating disease. During the week or so that the human embryo is traveling down the oviduct, it is simply a slowly dividing ball of undifferentiated cells. Until they implant, these cells have no different status than any other group of cells that have become separated from the rest of the tissues in the body, such as blood cells, skin cells etc.

In the abortion debate, general concensus on the principle that life begins at implantation could make legislation prohibiting abortion after implantation a lot more difficult to challenge in the courts. The majority opinion in Roe v. Wade acknowledged that "...if the fetus could be defined as a person for the purposes of the Fourteenth Amendment, then it would have a specific right to life under that amendment." The Roe opinion, in assessing the competing Fourteenth Amendment rights of the mother and the "entity" she carries, gave preference to the mother. Now, that "entity" would be able to compete on a more level playing field.

For a more comprehensive discussion on the subject of human beginnings, see adam