Help

Can someone help me out with the following.
I am having a hard time reading a birth certificatea from mid 1800's, I was under the assumption "hijo Natural" is a child born to a single mom!

what is the difference between:

Hijo Legitimo de - to - Hijo natural de
on both terms parents are listed as a couple.
In other cases the name of the baptised is listed followed by the term "Natural"
and then the name of the parents.

Help, Letty