At what level of development is a human foetus anatomically distinguishable from a cow foetus?
There's no fact-based reason to draw the line in any particular place. We, humanity, don't know what "personhood" really is beyond the laws we write while guessing and the just-so stories we tell each other to justify those laws.
That's why I'm vegetarian, and why I'd become vegan quickly as soon as someone can get milk from GM bacteria. (And sell it in supermarkets).
It's also one of two reasons why I try to be nice to LLMs: just in case. (The other reason takes it as read they have no experience of existence: by being trained on humans, they'll do better and worse exactly when real humans would do better and worse, and that means worse on holiday season and when getting insulted).