Originally, pregnancy was defined in terms of conception. For example, Webster''s Dictionary defines "pregnant" (or "pregnancy")
as "having conceived" (or "the state of a female who has conceived"), in its 1828 and 1913 editions.<7>
Both the 1828 and 1913 editions of Webster''s Dictionary said that to "conceive" meant "to begin the formation of the embryo." It was only in 1875 that Oskar Hertwig discovered that fertilization includes the penetration of a spermatozoon into an ovum. Thus, the term "conception" was in use long before the details of fertilization were discovered. By 1966, a more precise meaning of the word "conception" could be found in common-use dictionaries: "the formation of a viable zygote."<8>
In 1959, Dr. Bent Boving suggested that the word "conception" should be associated with the process of implantation instead of fertilization.<9> Some thought was given to possible societal consequences, as evidenced by Boving''s statement that "the social advantage of being considered to prevent conception rather than to destroy an established pregnancy could depend on something so simple as a prudent habit of speech." In 1965, the American College of Obstetricians and Gynecologists (ACOG) adopted Boving’s definition: "conception is the implantation of a fertilized ovum."<10>
The 1965 ACOG definition was imprecise because, by the time it implants, the zygote is called a blastocyst,<11> so it was clarified in 1972 to "Conception is the implantation of the blastocyst."<12> Some dictionaries continue to use the definition of conception as the formation of a viable zygote.<