Until the 1970s, medical professionals, human rights groups, and birth control providers traditionally understood Human Life to begin at conception (also termed "fertilization"):
Hippocratic Oath. The ancient Greek Hippocratic Oath – for thousands of years the standard for Western medical ethics – was routinely sworn by doctors upon medical school graduation.
It states: "I will neither give a deadly drug to anyone if asked for it, now will I make a suggestion to this effect. Similarly, I will not give to a woman an abortive remedy." This last sentence has now been deleted from most medical school commencement ceremonies.
American Medical Association. For 125 years, the American Medical Association took a firm anti-abortion position, declaring in 1859 that abortion is the "unwarranted destruction of human life." In 1871, the AMA denounced doctors who would perform abortions as "false to their professions, false to principle, false to honor, false to humanity, false to God." But, in 1989, the AMA called abortion a "fundamental right," to be decided "free of state interference" in the absence of compelling justification.
World Medical Association. Partially in response to revelations of medical war crimes at the Nuremberg Nazi trials, the World Medical Association in 1948 adopted a new physician's code, the Declaration of Geneva, which stated: "I will maintain the utmost respect for human life, from the time of conception." This declaration was reaffirmed in the 1970 Declaration of Oslo.
The United Nations. The United Nations Declaration on the Rights of the Child, adopted by the General Assembly in 1959, stated that a child "needs special safeguards and care, including appropriate legal protection, before as well as after birth." This is reaffirmed in the 1990 United Nations Convention on the Rights of the Child.
Planned Parenthood in 1963. Planned Parenthood insisted that the organization's birth control campaign did not support abortion, stating: "An abortion kills the life of a baby after it has begun."
California Medical Association. The California Medical Association, in 1970, declared abortion to be "killing" and referred to "the scientific fact, which everyone really knows, that human life begins at conception and is continous whether intra- or extra-uterine until death."
Scientific Community. Science has determined that at conception/fertilization a new individual comes into being, possessing a unique genetic code that has already determined that individual's sex, fingerprints, hair and eye color, facial features, etc.
In the 1960s, some in the scientific community who were proponents of abortion and artificial contraception, changed the definition of the word "conception" to mean the time the embryo implants in the endometrium (the lining of the uterus/womb). Fertilization still is defined as the union of sperm and egg to produce the zygote, the first cell of the new human being.
Pregnancy was also redefined to begin at implantation, not at fertilization.
By redefining conception, the period from fertilization until implantation, approximately 7-10 days, these semantics spinners were able to denote the very early, unique human embryo as a "pre-embryo", a non-living, non-human entity, in order to circumvent issues of early abortion and the abortifacient abilities of artificial contraception and emergency contraceptives, such as the Plan B.