When it comes to the question of when human life begins, the science is crystal clear: human life begins at conception or fertilization. Americans United for Life, in a legal briefing it has prepared for the Supreme Court, has put together a collection of citations from scientific literature that proves the point.
The citations are reproduced below and they include information from the latest scientific textbooks, papers and medical agencies.
A New, Distinct Human Organism Comes into Being at Fertilization
It is undisputed that a new, distinct human organism comes into existence during the process of fertilization. Scientific literature states the following:
• “The fusion of sperm and egg membranes initiates the life of a sexually reproducing organism.”
• “The life cycle of mammals begins when a sperm enters an egg.”
• “Fertilization is the process by which male and female haploid gametes (sperm and egg) unite to produce a genetically distinct individual.”
• “The oviduct or Fallopian tube is the anatomical region where every new life begins in mammalian species. After a long journey, the spermatozoa meet the oocyte in the specific site of the oviduct named ampulla, and fertilization takes place.”
• “Fertilization – the fusion of gametes to produce a new organism – is the culmination of a multitude of intricately regulated cellular processes.”
The government’s own definition attests to the fact that life begins at fertilization. According to the National Institutes of Health, “fertilization” is the process of union of two gametes (i.e., ovum and sperm) “whereby the somatic chromosome number is restored and the development of a new individual is initiated.” Thus, in the context of human life, a new individual human organism is initiated at the union of ovum and sperm. One textbook similarly explains: Human development begins at fertilization when a male gamete or sperm (spermatozoon) unites with a female gamete or oocyte (ovum) to produce a single cell – a zygote. This highly specialized, totipotent cell marked the beginning of each of us as a unique individual.
Thus, a new human organism is created before the developing embryo implants in the uterus – i.e., before that time at which some people consider a woman “pregnant.”
 See, e.g., Condic, When Does Human Life Begin? A Scientific Perspective (The Westchester Institute for Ethics & the Human Person Oct. 2008), https://bdfund.org/wordpress/wpcontent/uploads/2012/06/wi_whitepaper_life_print.pdf; George & Tollefsen, EMBRYO 39 (2008).
 Marsden et al., Model systems for membrane fusion, CHEM. SOC. REV. 40(3):1572 (Mar. 2011) (emphasis added).
 Okada et al., A role for the elongator complex in zygotic paternal genome demethylation, NATURE 463:554 (Jan. 28, 2010) (emphasis added).
 Signorelli et al., Kinases, phosphatases and proteases during sperm capacitation, CELL TISSUE RES. 349(3):765 (Mar. 20, 2012) (emphasis added).
 Coy et al., Roles of the oviduct in mammalian fertilization, REPRODUCTION 144(6):649 (Oct. 1, 2012) (emphasis added).
 Marcello et al., Fertilization, ADV. EXP. BIOL. 757:321 (2013) (emphasis added).
 National Institutes of Health, Medline Plus Merriam-Webster Medical Dictionary (2013), https://www.merriamwebster.com/medlineplus/fertilization (emphasis added).
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 Moore & Persaud, THE DEVELOPING HUMAN 16 (7th ed. 2003) (emphasis added).
 For an overview of how the definition of “pregnancy” has changed, see Gacek, Conceiving Pregnancy: U.S. Medical Dictionaries and Their Definitions of Conception and Pregnancy, FRC INSIGHT PAPER (Apr. 2009), https://downloads.frc.org/EF/EF09D12.pdf.