Is there a scientific reason for that idea? I don't see how implantation can really be considered a transition from potential human to human. I think with enough scientific advances a human could be raised entirely outside the womb, and in that case there would be no implantation.
(Implantation itself doesn't feel like the transition point either, but it's just the last obvious discrete step before the continuous progression all the way to birth.)
It seems strange to say that our current definition is not the correct long-term definition, but we'll keep using the current one. To me that's basically saying our current definition of human is wrong, and I don't see why it's acceptable to use a wrong definition.
I'm not sure it avoids the majority since a decent amount of folks have the knowledge and tools to simply avoid the pregnancy in the first place, but it does lower it.
Putting a newborn up for adoption is different, it doesn't have either of those problems. There are 2 million families in the US waiting to adopt a newborn.