John Gill Commentary Ruth 4:16

"And Naomi took the child, and laid it in her bosom, and became nurse unto it." — Ruth 4:16 (ASV)

And Naomi took the child, and laid it in her bosom
As a token of her most tender love and affection for it; this it is probable she did quickly after the birth of it:

and became a nurse unto it;
that is, after the mother had suckled and weaned it, then she took it from her, and brought it up.