I have no knowledge about this myself, but it’s an interesting take, from a reader:

It was interesting that you posted an email where the poster who claims embryos aren’t human as they haven’t yet implanted. Up until the mid 1950s the consensus of science was that life began when the egg was fertilized. So why the change in the 50s? Simple-it was the development of the IUD for birth control. Since it works by preventing implantation the pharmaceutical companies started a massive PR campaign to change the definition of conception from when the egg was fertilized to when it was implanted. obviously they were very successful.