You should treat this fact as an absolutely certainty. It's not obscure and ambiguous. It's clear, simple and certain. All doubt about it is born of malice and irresponsibility, and it's always accompanied by a bad conscience. Humans don't conceive dogs or cats or fingernails or appendices or other parts of humans or potential humans.
Humans conceive and give birth to humans, pure and simple.
We must reject all talk of degrees of humanity as the bigotry it is. Still, after all I've said, some Christians will seek to justify their lax attitude toward abortion by pointing to the differences between preborn human life and post birth humanity. These observations don't really deserve a reply. They're malignant attempts to define humanity in a way that excludes the preborn. And we should make this very clear to those who make them.