In 1965, the medical definition for when pregnancy begins was changed by the American College of Obstetricians and Gynecologists (ACOG). Why? Ideology.
At the 1959 Planned Parenthood/Population Council symposium, Dr. Bent Boving argued for changing the definition by moving the date of conception from when fertilization occurs to when implantation occurs. He said that “the social advantage of [birth control] being considered to prevent conception rather than to destroy an established pregnancy could depend upon something so simple as a prudent habit of speech.”1
In 1964, Dr. Christopher Tietze pushed this propaganda further. After noting that many religious and legal experts accept medical consensus as fact, he said that “if a medical consensus develops and is maintained that pregnancy, and therefore life, begins at implantation, eventually our brethren from the other faculties will listen.”2 Tietze would later win the Planned Parenthood Federation of America Margaret Sanger Award for outstanding contributions to the pro-abortion movement.
Ironically, medical textbooks STILL teach the more traditional definition of pregnancy:
Medical textbooks still teach that human life begins at conception when sperm and egg unite. See Keith L. Moore, T.V.N. Persaud, The Developing Human: Clinically Oriented Embryology (Philadelphia: Saunders, 2008). Keith L. Moore is a recipient of the American Association of Anatomists’ highest award for excellence.
From the Guttmacher Institute:
To be sure, not every act of intercourse results in a pregnancy. First, ovulation (i.e., the monthly release of a woman's egg) must occur. Then, the egg must be fertilized. Fertilization describes the process by which a single sperm gradually penetrates the layers of an egg to form a new cell ("zygote"). This usually occurs in the fallopian tubes and can take up to 24 hours. There is only a short window during which an egg can be fertilized. If fertilization does not occur during that time, the egg dissolves and then hormonal changes trigger menstruation; however, if fertilization does occur, the zygote divides and differentiates into a "preembryo" while being carried down the fallopian tube toward the uterus. Implantation of the preembryo in the uterine lining begins about five days after fertilization. Implantation can be completed as early as eight days or as late as 18 days after fertilization, but usually takes about 14 days. Between one-third and one-half of all fertilized eggs never fully implant. A pregnancy is considered to be established only after implantation is complete.
Source: American College of Obstetricians and Gynecologists.
There's a LOT more at that site - I suggest that you check it out when you have a few minutes.
This is a MEDIA fight - a fight to ensure that all sources of information for the public sing the same tune. A full discussion is here. And more here (from a Catholic viewpoint).