The beginning of human personhood is a concept framed by varied arguments proposing different points in the biological development of human life as the beginning of when such life is to be considered to be morally and legally a human person. While biology can illuminate the issue, personhood is a political, philosophical, and religious concept that impacts controversial issues such as the abortion debate, the stem cell controversy, reproductive rights, and fetal rights.
Answers to the question of when human life begins have varied among social contexts, and have changed with shifts in ethical and religious beliefs, often as a result of advances in scientific knowledge; in general they have developed in parallel with attitudes to abortion.
Plato, and likewise the Stoics, held the view that ensoulment did not happen until birth. Aristotle stated a view that abortions should happen only early in pregnancy, before certain biological processes began. He first proposed the connection between ensoulment, what he called "animation", and discernible biological properties. While the early Catholic Church had not yet defined when ensoulment occurs, it has always opposed abortion as a mortal sin, no matter when in the pregnancy it was enacted; the Church has since defined that life (ensoulment) occurs at conception after consideration of modern evidence.
Aristotle believed that males and females were ensouled at different times, males before females, according to a stereotype that they had a different degree of vigour for life. Before the Catholic Church considered modern evidence and defined when ensoulment occurs, St. Thomas Aquinas and St. Augustine of Hippo held the view that fetuses were "animated" (using Aristotle's term for ensoulment) near the 40th day after conception. The Church, as it always has for nearly 2,000 years, condemns the procuring of abortion as a grave sin.
The Pythagoreans, who claimed that the human soul was created at that time, also opposed abortion. This is reflected in the Hippocratic oath which forbids abortion along with infanticide: "Hippocrates was of seemingly a minority position in ancient Greece, in that he disapproved of abortion. The Oath expressly forbids giving a woman 'an instrument to produce abortion', and it has been interpreted to forbid inducing abortion by any other method."
Philosophers of the 1960s and 70s, for example Judith Jarvis Thomson and Ayn Rand, have presented the view that the rights and health of the pregnant woman must be considered to supersede any rights the fetus may be held to have. Rand stated that "an embryo has no rights". 
A basic requirement for personhood is individuality, which entails differentiation between the person and its parents. Biology offers a number of stages in the life cycle that have been seen as candidates for personhood:
Fertilization is the fusing of the gametes, that is a sperm cell and an ovum (egg cell), to form a zygote. At this point, the zygote is genetically unique from either of its parents. Many members of the medical community accept fertilization as the point at which life begins. Dr. Bradley M. Patten from the University of Michigan wrote in Human Embryology that the union of the sperm and the ovum “initiates the life of a new individual” beginning “a new individual life history.” In the standard college text book Psychology and Life, Dr. Floyd L. Ruch wrote “At the time of conception, two living germ cells—the sperm from the father and the egg, or ovum, from the mother—unite to produce a new individual.” Dr. Herbert Ratner wrote that “It is now of unquestionable certainty that a human being comes into existence precisely at the moment when the sperm combines with the egg.” This certain knowledge, Ratner says, comes from the study of genetics. At fertilization, all of the genetic characteristics, such as the color of the eyes, “are laid down determinatively.” James C. G. Conniff noted the prevalence of the above views in a study published by the New York Times Magazine in which he wrote, “At that moment conception takes place and, scientists generally agree, a new life begins—silent, secret, unknown.”
The view that life begins at fertilization reached acceptance from mainstream sources at one point. In 1967, New York City school officials launched a large sex education program. The fifth grade text book stated “Human life begins when the sperm cells of the father and the egg cells of the mother unite. This union is referred to as fertilization. For fertilization to take place and a baby to begin growing, the sperm cell must come in direct contact with the egg cell.” Similarly, a text book used in Evanston, Illinois stated: “Life begins when a sperm cell and an ovum (egg cell) unite.” Catholic philosopher Peter Kreeft goes so far as to say:
Well, every biology textbook in the world, before Roe v. Wade, was not in doubt in answering the question, "When does an individual life of any mammalian species begin?" The answer is, "When the genetic code is complete." When instead of the haploid ovum and the haploid sperm, you get the diploid embryo. And at that point, something happens that is totally different, because the thing that's there seems totally different.
One objection raised to the fertilization view is that not all of the objects created by the union of a sperm and an egg are human beings. Objects such as hydatidiform moles, choriocarcinomas, and blighted ovums are clearly not. Neither will every normal zygote will develop into an adult. There are many fertilized eggs that never implant and are “simply washed away” after conception, though this can be answered by the fact that not every child becomes an adult; organisms die at various developmental stages.
The unique genetic identity of the zygote is also challenged. In fertilization, chromosomes from each parent are combined in the same cell nucleus but remain independent; every chromosome in a diploid cell can be traced to one parent and not the other. Only during meiosis, in which gametes are formed, do these chromosomes cross over, exchanging bits of DNA to form unique genes not found in either parent, though this objection would also apply to the genome of an adult. However, gametes are not commonly considered to have personhood, perhaps because most of them are never involved in fertilization.
That a human individual's existence begins at conception is the accepted position of the Roman Catholic Church, whose Pontifical Academy for Life declared: "The moment that marks the beginning of the existence of a new 'human being' is constituted by the penetration of sperm into the oocyte. Fertilization promotes a series of linked events and transforms the egg cell into a 'zygote'." The more authoritative Congregation for the Doctrine of the Faith also has stated and reaffirmed: "From the time that the ovum is fertilized, a new life is begun which is neither that of the father nor of the mother; it is rather the life of a new human being with his own growth." Eastern Orthodox churches and most of the more conservative Protestant denominations also teach this view of life.
Biochemically, this is when alpha announces its presence as part of the human community by means of its hormonal messages, which we now have the technology to receive. We also know biochemically that it is an independent organism distinct from the mother. (Note: in writing the book, "alpha" was Nathanson's term for any human before birth.)
For fourteen to twenty-one days after fertilization, an embryo may segment and form twins, triplets, etc. Some argue that an early embryo cannot be a person because "If every person is an individual, one cannot be divided from oneself."
However, Fr. Norman Ford stated that "the evidence would seem to indicate not that there is no individual at conception, but that there is at least one and possibly more." He went on to support the idea that, similar to processes found in other species, one twin could be the parent of the other asexually. Theodore Hall agreed with the plausibility of this explanation saying, "We wonder if the biological process in twinning isn't simply another example of how nature reproduces from other individuals without destroying that person's or persons' individuality."
Until the fetus is viable, any rights granted to it must come at the expense of the pregnant woman, simply because the fetus cannot survive except within the woman's body. Upon viability, the pregnancy can be terminated, as by a c-section or induced labor, with the fetus surviving to become a newborn infant. Several groups believe that abortion before viability is acceptable, but is unacceptable after. In some countries, early abortions are legal in all circumstances, but late-term abortions are limited to circumstances where there is a clear medical need.
The Jewish Talmud holds that a fetus's life is less valuable than a woman's; if the woman's life is endangered by the pregnancy, it requires an abortion. However, if the "greater part" of the fetus has emerged, then its life may not be taken even to save the mother's, "because you cannot choose between one human life and another".
Some Christian theologians hold that ensoulment occurs when an infant takes its first breath of air. They cite, among other passages, Genesis 2:7, "And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul."
There are also other ideas of when personhood is achieved:
In 1973, Harry Blackmun wrote the court opinion for Roe v. Wade, saying "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."
In 2003, the Partial-Birth Abortion Ban Act was enacted, which prohibits an abortion if "either the entire baby's head is outside the body of the mother, or any part of the baby's trunk past the navel is outside the body of the mother."