> Well if we go to an even more extreme degree with the devil’s advocate thing: what is the difference between an ~4 month embryo (or any cutoff after which miscarriage becomes very unlikely) and a newborn?

From a practical perspective, giving up the newborn for adoption is significantly easier than the 4 month embryo - it's already been carried to term and birthed. And quite a few humans, correctly or otherwise, feel a lot more 'ick' around the idea of infanticide vs. abortion. But they also feel a lot more 'ick' around late gestation abortion too - most states have had gestational limits around abortion. Very few have had none. (Many of those with gestational limits do have exceptions around health/safety issues for the mother, or significant medical issues with the fetus, etc.)

It's kind of a strange argument because most of the US did accept the premise that at a certain point in the embryo's development abortion becomes less acceptable. I can't speak as definitively for the rest of the world, but a cursory google shows that the situation is quite similar - most countries that allow for abortion have gestational restrictions.