His son by the slave woman was born as the result of human effort, but his son by the free woman was born as the result of a divine promise. (NIV)
But whereas the child of the slave woman was born according to the flesh and had an ordinary birth, the son of the free woman was born in fulfillment of the promise. (AMP)
But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. (KJV)