I haven't seen the Federal definition in any official document, but numerous pro-abortion editorials say that the definition is, specifically, implantation in the endometrium.
So an ectopic pregnancy isn't pregnancy?
Oh, and how about this bit of reasoning, from a "Catholic" source:
Now, just because an egg is fertilized doesn't necessarily mean that it will develop into an embryo. For that to happen, the fertilized egg must be implanted into the endometrium that lines the uterus. Implantation happens seven days after fertilization, if it happens at all. ...
For this reason, according to the medical definition, a woman is not considered pregnant until the developing embryo successfully implants the lining of the uterus.[emphasis added]
Pardon, NCR, but according to you an embryo can't implant, successfully or otherwise, because there's no such thing as an embryo until the implantation occurs.