There is no "one less than the value chosen" in this case - the embryo is 100% reliant on the mother for every single possible life function - circulation, respiration, digestion, homeostasis, everything. The only "babies" that are "born" in that state are early miscarriages. This is the slippery slope fallacy writ large, in the same way as comparing a minority to an embryo was.
You think that a 20 week old child has no chance of living outside a womb? The current earliest premie is 21 weeks. It will only go lower with better technology. The slope slips to its logical conclusion: conception.
Here’s a thought exercise: take whatever you would consider a viable human and rewind to one developed cell earlier. Is that a human? Repeat until you can guarantee it is not a human. How far do you have to go until that is true for all instances of all fetuses? I came to the conclusion that an embryo, which can live and grow outside of a womb is the earliest logical point to consider the organism a human and make subsequent opinions accordingly.
