George Haydock's Catholic Commentary
Psalms 138:13
Reins, and interior. (Theodoret) --- Nothing seems more hidden than a man's entrails, or a child in his mother's womb, who is formed by God, ver. 16., and 2 Machabees vii. 22. (Worthington) --- Protected. Hebrew, "covered," which may also mean formed. (Berthier)