“Mainstream” and “more” are not scientific terms. They do not convey scientific truthfulness. Truth does not bend to democracy.
It is interesting that Catholics now update their truths with what is discovered scientifically. Now Catholics believe that human conception actually happens seconds after the union of human sperm and egg? Very scientific!
Viability is surety of something to continue. It means, what was to continue is already there. For the case of implantation as a sign of viability, it would be viability that life will continue. Therefore, life should have started before this event. The claim is that it started at fertilization — the union of sperm and egg. Makes sense to me.