It's not that "they" have defined pregnancy as beginning at implantation. That's simply the biological reality. Pregnancy is a condition of a woman, not of a fertilized egg or embryo.
As demonstrated by IVF, conception can occur outside the body, and in fact the embryo can develop outside the body to as far as the blastocyst stage without any ill effects, because it is inherently an independent entity through that stage. Nobody is pregnant when an embryo is in a petri dish dividing up to the blastocyst stage. At that stage (with or without an interruption of decade or more, while it sits in a freezing nitrogen tank), you could still randomly transfer it into any one of a group of women whose uterine linings are at the right stage, and have an equal chance of any one of those *women* becoming pregnant. But until it forms a functioning connection to a uterine lining, none of the women are pregnant.
While I don't share the views of most here, that early embryos should be regarded as full-fledged human beings, I respect intelligent arguments in support of that position. What I don't respect is people throwing around baseless accusations in an attempt to promote that position. Defining pregnancy as beginning at implantation is not some conspiracy by people who don't think embryos should be considered as being on a par with fully developed humans who have been born and no longer have or require a physical connection to their birth mother. It's a biological fact that is fully understood, accepted, and taught by all well-informed biologists and physicians, including those who *do* believe that a just-fertilized egg has the ethical status of, and should have the legal status of a young child.
Beginning of pregnancy controversy
http://en.wikipedia.org/wiki/Beginning_of_pregnancy_controversy
More:
However, as often is the case in contentious issues, such as abortion and contra-ception, what is clear is made ambiguous. The dirty little word game is that when Family Planning advocates use the word “conception,” they mean the entire process from when the egg and ovum combine (fertilization) through the point at which the fertilized ovum attaches to the wall of the mother’s uterus (implantation). Under this definition, a “contraceptive” cannot only prevent sperm and ovum from meeting (as a condom clearly would) but also prevent a fertilized ovum from attaching to the uterine wall.
The word game here is that as long as the fertilized ovum has not attached to the uterus, the mother technically is not yet pregnant. Thus defined, some, hormonal contraceptives (including Barr Pharmaceutical’s “Plan B”) and Inter Uterine Devices (IUDs) simply “prevent pregnancy.” At least Barr Pharmaceutical’s web site is forthright in stating “Plan B may also work by preventing it [the fertilized ovum] from attaching to the uterus (womb).”
http://www.pewsitter.com/view_news_id_10196.php
Guess what? We no longer have to be subjected to your views!