Definitions of natural child
a biological child of the parent being discussed
I find it no coincidence that this is the one child under your care that was not your natural child.
a child born to parents who are not married to each other
A natural father or mother shall have the power to legitimate his or her natural children.
a child born to parents who are not married to each other who is acknowledged by its father
Natural children can take from their father by intestate succession "to the exclusion only of the State".