Albert Barnes Commentary Psalms 139:13

"For thou didst form my inward parts: Thou didst cover me in my mother`s womb." — Psalms 139:13 (ASV)

For you have possessed my reins—The word here rendered “possessed” properly means to “set upright,” to “erect,” and therefore, the derivative of the verb is applied to a cane or reed, as being erect. Then the word means to found, to create (Genesis 14:19, Genesis 14:22)—as the heavens and the earth; and then, to get, to gain, to purchase, etc. Here the word seems to be used in its original sense: to make, create, etc. The idea is not, as in our translation, that God “possessed” or “owned” them, but that he had “made” them and, therefore, he knew all about them.

The word “reins” literally means the “kidneys”; and then, it comes to denote the inward part, the mind, the soul, the seat of the desires, affections, and passions (Jeremiah 11:20). See (Psalms 7:9), note; (Job 19:27), note.

The meaning here is that God had made him; that the innermost recesses of his being had been constituted as they are by God; and that, therefore, he must be able to see all that there is in the very depths of the soul, however it may be hidden from the eye of man.

You have covered me in my mother’s womb—The word here rendered “cover” properly means to interweave, to weave, to knit together. The literal translation would be, “You have woven me in my mother’s womb,” meaning that God had put his parts together, as one who weaves cloth or who makes a basket. So it is rendered by DeWette and by Gesenius (Lexicon).

The original word, however, also has the idea of protecting, as in a booth or hut, woven or knit together—namely, of boughs and branches. The former meaning best suits the connection. The sense would then be that as God had made him—as he had formed his members and united them in a bodily frame and form before he was born—he must be able to understand all his thoughts and feelings.

As he was not concealed from God before he saw the light, so he could not be anywhere.