Quite simply, the time of origination and the time of birth are two different points in time.
The answer is both. You are free to think that, but you are naive or ill informed by doing so.
We know all forms of life by what they are and by what they are not. For example, we know a whale is not a fish because it cannot extract oxygen from the water in which it swims and because it's tail bones are horizontally oriented in relation to the rest of its skeleton. We know the whale is a mammal because it has all of the defining characteristics of mammals.
Quite frankly, for the sake of discussing the matter at hand, I don't care what the law says. Laws have said plenty that's been found to be inaccurately rationalized and concluded upon before. What the law says on the matter of personhood is as far as I'm concerned irrational on the matter of personhood, not just with regard to human personhood, but also with regard to corporate personhood.
Other:
What you failed to realize through all of that is that I have my own definition of what is and is not a person. I do not accept the definitions and lines of argument you've presented because I find them unacceptable. I don't care whether one or a million scientists think a fetus is also a human being. I don't and that's in part what drives my views on the matter.
Can you find credible scientist who'll assert that personhood begins before birth? Of course you can. The fact of the matter is that there are multiple positions within science on when life begins:
- Metabolic View:
The metabolic view takes the stance that a single developmental moment marking the beginning of human life does not exist. Both the sperm and egg cells should individually be considered to be units of life in the same respect as any other single or multicellular organism. Thus, neither the union of two gametes nor any developmental point thereafter should be designated as the beginning of new life.
- Genetic View:
The genetic view takes the position that the creation of a genetically unique individual is the moment at which life begins. This event is often described as taking place at fertilization, thus fertilization marks the beginning of human life. During this developmental event, the genes originating from two sources combine to form a single individual with a different and unique set of genes. One of the most popular arguments for fertilization as the beginning of human life is that at fertilization a new combination of genetic material is created for the first time; thus, the zygote is an individual, unique from all others.
- Embryological View:
In contrast to the genetic view, the embryological view states that human life originates not at fertilization but rather at gastrulation. Human embryos are capable of splitting into identical twins as late as 12 days after fertilization resulting in the development of separate individuals with unique personalities and different souls, according to the religious view. Therefore, properties governing individuality are not set until after gastrulation. This view of when life begins has also been adopted as the official position of the British government. The implications of a belief in this view include giving support to controversial forms of contraception including the "morning after" pill and contra-gestational agents as long as they are administered during the first two weeks of pregnancy.
- Neurological View:
Although most cultures identify the qualities of humanity as different from other living organisms, there is also a universal view that all forms of life on earth are finite. Implicit in the later view is the reality that all life has both a beginning and an end, usually identified as some form of death. The debate surrounding the exact moment marking the beginning of a human life contrasts the certainty and consistency with which the instant of death is described. Contemporary American (and Japanese) society defines death as the loss of the pattern produced by a cerebral electroencephalogram (EEG). If life and death are based upon the same standard of measurement, then the beginning of human life should be recognized as the time when a fetus acquires a recognizable EEG pattern. This acquisition occurs approximately 24- 27 weeks after the conception of the fetus and is the basis for the neurological view of the beginning of human life.
- Ecological View:
Advocates of the neurological view contend that human life begins when a developing fetus acquires humanness, a point designated by brain activity that can be described as characteristically human. But if this developing fetus is separated from its mother at an early stage, regardless of the state of neural development, the fetus will be unable to sustain life on its own. The total dependence of the developing fetus for the majority of gestation catalyzed the formation of another view of when human life begins. The ecological/technological view of when human life begins designates this point when an individual can exist separately from the environment in which it was dependent for development (i.e., its mother's womb).
- Philosopher's Conundrum:
There are philosophers, although not very many, who would dare to make the stance that a fetus nor an infant is a human being because it does not possess a consciousness of itself. This of course means that neither a zygote nor an embryo is a person either. Michael Tooley is one of these philosophers who describes his perspective in the article "Abortion and Infanticide." Essentially he argues that abortion and infanticide are really no different, if you support one, then you must support the other. His argument is that in order to claim that an adult has the right to live and an embryo or a fetus does not, one must be able to identify some moment where the moral status of the organism in question changes. There is nothing inherent about birth that it should automatically be hailed as this defining moment. A more justified moment, Tooley argues, is the moment at which the human child gains consciousness. At this moment, not at birth, should the child be considered a full fledged person, entitled to all the rights, particularly the right to life, that human adults are entitled to.
The main problem that most people find with this position on when human life begins is that it condones infanticide, arguing that infants do not have the same right to life as adult humans do. Must people reject this view of when life begins, finding it impossible to support a view that logically leads to the conclusion that infanticide is acceptable. Tooley, however, argues that this rejection of his perspective is based on a purely emotional response to the idea of infanticide and not on logic or reasoning.
Historically, the question of when human life begins was answered by a progression that was initiated by edicts on abortion which were governed by the popular notions of moral acceptability. These popular notions were decrees put forth by God, delivered to the populous through religious texts. Modern technological innovations of the twentieth century have reversed the order of this progression; contemporary scholars often address the question of when human life begins by first evaluating scientific data. The conclusions reached via the scientific method become the tools used to create popular standards of moral acceptability. These contemporary notions of moral acceptability then provide the framework for the modern abortion debate.
The temporal divergence between the progressions of thought leading to answers of when human life begins reveals a shift in the source of knowledge that is used to answer one of humanity's most puzzling questions. Prior to the twentieth century, God was humanity's source of absolute knowledge. In recent years, however, scholars have terminated the utility of God's omniscience and in its place have raised science and technology as their source of absolute knowledge. This shift is evidence for, perhaps, the most determinant factor of any argument for when human life begins. The reasons governing the variation in both historical and modern views of when life begins is largely due to a variation in moral standards.
However, understanding the basis for societal moral standards appears to be the key to discerning how to approach the question of when human life begins. Science has not been able to give a definitive answer to this question. One opinion is that the acquisition of humanness is a gradual phenomenon, rather than one that occurs at any particular moment. If one does not believe in a "soul," then one need not believe in a moment of ensoulment. The moments of fertilization, gastrulation, neurulation, and birth, are then milestones in the gradual acquisition of what it is to be human. While one may have a particular belief in when the embryo becomes human, it is difficult to justify such a belief solely by science.
Parting thoughts:
The thing I find noteworthy in this discussion with you is that the question you asked is when personhood begins and I happen to think that personhood and life are not the same things. Life clearly can and does exist without that life being a person. A human fetus is no exception. It is not a human being; it is a human fetus or embryo. Is it alive enough to call it so? Probably yes, but alive or not, it is yet a proto-human, not a human; thus it is also not a person.
Most importantly, however, that there are talented and learned folks on both sides of this issue informs me that more likely the matter isn't one well or best decided by science until such time as science agrees on a single "truth" about the matter. You see you and and scientists you cited are arguing based on whether an embryo/fetus is alive. In contrast my answer derives from what I think is a person/human being, which is what you asked about.
The simple fact is that I have no issue with person electing to terminate the existence of proto-person provided the proto-person belongs to the person who makes that decision. I have no need to encourage one from doing so, but neither have I a need to discourage one from doing so. I don't have to live with their choice. I have enough worries of my own without having to add to them the welfare of a proto-human for which I had zero role in effecting it on the path toward becoming a person.
