For you have formed my inward parts: you have covered me in my mother's womb.
Read Chapter 139
Augustine of Hippo
13. "For Thou, O Lord, hast possessed my reins" (ver. 13). The Possessor is within; He occupieth not only the heart, but also the reins; not only the thoughts, but also the delights: He then possesseth that whence I should feel delight at any light in this world: He occupieth my reins: I know not delight, save from the inward light of His Wisdom. What then? Dost thou not delight that thy affairs are very prosperous, times fortunate to thee? dost thou not delight in honour, in riches, in thy family? "I do not," saith he. Wherefore? Because "Thou hast possessed my reins, O Lord; Thou hast taken me up from my mother's womb." While I was in my mother's womb, I did not regard with indifference the darkness of that night and the light of that night. ...Now, having been taken up froth the womb of that our mother, we look on them with indifference, and say, "As is His darkness, so is also His light." Neither doth earthly prosperity make us happy, nor earthly adversity wretched. We must maintai...
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)