"To begin with, scientifically something very radical occurs between the processes of gametogenesis and fertilization the change from a simple part of one human being (i.e., a sperm) and a simple part of another human being (i.e., an oocyte usually referred to as an "ovum" or "egg"), which simply possess "human life", to a new, genetically unique, newly existing, individual, whole living human being (a single-cell embryonic human zygote). That is, upon fertilization, parts of human beings have actually been transformed into something very different from what they were before; they have been changed into a single, whole human being. During the process of fertilization, the sperm and the oocyte cease to exist as such, and a new human being is produced." - Dianne N. Irving, M.A., Ph.D. - International Journal of Sociology and Social Policy 1999
"[T]hrough the mingling of maternal and paternal chromosomes, the zygote is a genetically unique product of chromosomal reassortment, which is important for the viability of any species." - Bruce M. Carlson, Human Embryology and Developmental Biology (St. Louis, MO: Mosby, 1994)
"...scientifically there is absolutely no question whatsoever that the immediate product of fertilization is a newly existing human being. A human zygote is a human being. It is not a "potential" or a "possible" human being. It's an actual human being with the potential to grow bigger and develop its capacities." - Dianne N. Irving, M.A., Ph.D. - International Journal of Sociology and Social Policy 1999
"Though there has been at least one report of brain activity occurring as early as seven weeks into pregnancy, researchers have determined that the first electroencephalogram (EEG) activity usually occurs in the brainstem at around 10 weeks of gestational age." - Bergstrom RM, Bergstrom L. Prenatal development of stretch reflex functions and brainstem activity in the human. Ann Chir Gynaecol Fenn1963;52 (suppl) :1–21S.
"Although it would hence not be entirely unreasonable, as an epistemic matter, to infer that the fetus cannot be a person prior to the beginning of higher brain activity, there is a moral reason for caution. After all, if the instantiation of a self in a human fetus is a sufficient condition for moral personhood, the moment at which the fetus instantiates a self marks the beginning of the period during which the moral impermissibility of abortion rises to the level of murder.
Accordingly, moral considerations may dictate acting on (as opposed to accepting) a more conservative conclusion about fetal personhood; for allowing abortion after 10 weeks, given the epistemic uncertainty associated with fetal personhood, runs a substantial risk of resulting in murder. Hence, even if there are reasonable epistemic grounds for believing that the selfhood/ensoulment cannot begin until the first episodes of cortical brain activity, there are strong moral grounds that preclude basing the law on such a belief.
Either conclusion, however, entails reasonably extensive abortion-rights. Because, according to Centers for Disease Control (CDC) figures, 91% of all abortions are performed within the first trimester,29 a law that permitted abortions up to the beginning of discontinuous cortical activity would have the effect of allowing abortions up to 20 weeks and hence permit all first trimester abortions and then some. Further, because, according to CDC figures, 74.5% of all abortions are performed within the first 10 weeks, a law that permitted abortions up to the beginning of brainstem activity would have the effect of allowing most abortions up to 10 weeks.29xxvi In either case, a substantial number of abortions would be allowed under a law that permitted abortion until the beginning of the relevant form of brain activity." - Bergstrom RM, Bergstrom L. Prenatal development of stretch reflex functions and brainstem activity in the human. Ann Chir Gynaecol Fenn1963;52 (suppl) :1–21S.
"When Do Human Beings Begin? Scientific Myths and Scientific Facts" https://www.princeton.edu/~prolife/articles/wdhbb.html
"A dualist analysis of abortion: personhood and the concept of self qua experiential subject" http://m.jme.bmj.com/content/31/1/48.full