Nor does it have to be a brain or a consciousness. It's developing.
Yes, it's developing. At some point it will develop in to a human being. A collection of cells with no brain is not a human being.
What differentiates a 23 week baby from a 24 week one?
See:
The cortex, which is responsible for human consciousness, does not develop until 24-26 weeks.
The baby isn't a person when it's 23 weeks, 6 days, 23 hours and 59 seconds old but when it's 24 weeks old, in that magical second, it became a person.
You can't become a person in one day.
How would anyone legally define this?
What is a person anyways?
Merriam-Webster
Definition of person
1: HUMAN, INDIVIDUAL sometimes used in combination especially by those who prefer to avoid man in compounds applicable to both sexes
A person is a human. Human life is - human.