But That's None Of My Business

Of course they are

Because they're the product of a mother and father

Then define it, in your own words

So, you're saying that someone must be fully-formed to be considered a child, and therefore a person.
But what about the people with brain issues so bad they're just a vegetable, these people are still considered a person even though they're not fully formed

Fewer: completely or entirely; to the furthest extent.
Formed (in this context): gradually appear or develop
So, fully-formed means a COMPLETELY developed human. So, if you define fully-formed as if they are in the womb or not, you are being inaccurate, as that is not what it means. I am talking about people born with some kind of flaw that makes their brain not develop as much, effectively making them as helpless as fetuses.
If it's illegal to kill THEM
Why is it legal to kill FETUSES?