Ok, but we have words from all sorts of languages in English that don't mean the same thing as they originally did.

Fetus still means offspring, its meaning hasn't changed. One source: "An unborn offspring of a mammal, in particular an unborn human baby more than eight weeks after conception." Another source other source: "A developing human from usually two months after conception to birth."

It's Joe Public that doesn't know its definition, thinking its definition differs substantially from the English equivalents "offspring" or "baby," when it doesn't. Joe Public doesn't know this because "for some reason" we drop English when species homo sapiens is concealed on the wrong side of the mons pubis.

And I suppose you could call a fetus an undeveloped child if you also wanted to call a child an undeveloped adult,

A child is an undeveloped adult.

and afford it all the rights of a full-grown person.

"All the rights?" No. Right to life? Sure, I guess we can give undeveloped adults a right to life. But why? We kill undeveloped children when they're hidden inside a woman's pelvis, why not kill them outside it? The bone doesn't grant human rights. Peter Singer's right to call this distinction absurd. The Romans practiced abortion and infanticide, zero reason (by modern reasoning) why this country shouldn't either.

But that doesn't make any sense, does it?

It makes no sense any of this is even under discussion, yet here we are. Abortion is one of the most idiotic things this country has done and I look forward to seeing our descendants denouncing us for it allowing it when they're on their high horses, the same way we right now denounce past idiocies Americans have allowed. Toodles.