Medical science defines pregnancy as after implantation.
As we've seen from Tabor's, textbooks & dictionaries often aren't written by medical professionals, much less doctors.
Women who are planning a pregnancy will take their temps in order to predict ovulation and pregnancy. Many women know almost from the moment of conception, they are pregnant.
It becomes a problem for bc manufacturers if their products are terminating pregnancies rather than 'preventing' them. Most bc pills are preventative which means they prevent ovulation and fertilization. Once the egg has been fertilized, the woman IS pregnant and her body responds immediately to this biological change. If she is on the pill, there is yet another back up system....to keep the embryo from implanting. That is where the pill aborts the embryo by creating a hostile environment so it can't implant or remain implanted. Additionally, in cases of ectopic PREGNANCY a live birth can still occur, although rare. There are also cases where the embryo implants but is expelled soon after- which again- is an early abortion.
The definition is correct but due to the prolife fanatics, certain groups have to juggle semantics around in order to avoid lawsuits. You need to apologize because you were wrong and the other poster was right.