Douay version of Job 31:18:
(For from my infancy mercy grew up with me: and it came out with me from my mother's womb:)
St. Jerome's Latin Vulgate:
(quia ab infantia mea crevit mecum miseratio, et de utero matris meae egressa est mecum)
Yet other translations mention both widows and the fatherless/orphans in this verse.
The whole chapter is Job defending his own righteous behavior before God as regards others:
If I have walked with vanity, or if my foot hath hasted to deceit; Let me be weighed in an even balance, that God may know mine integrity. - Job 31:5-6
He lists his treatment of his wife (v. 7-12), his servants (v. 13), and the poor, widowed, and fatherless (v. 16-17, 19-21).
Parenthetically, Job is saying that he took care of him as with a father. Him here is most naturally the fatherless and he argues that he has shown such mercy from his mother's womb:
If I have withheld the poor from their desire, or have caused the eyes of the widow to fail; Or have eaten my morsel myself alone, and the fatherless hath not eaten thereof; (For from my youth he was brought up with me, as with a father, and I have guided her from my mother's womb;) If I have seen any perish for want of clothing, or any poor without covering; If his loins have not blessed me, and if he were not warmed with the fleece of my sheep; If I have lifted up my hand against the fatherless, when I saw my help in the gate: Then let mine arm fall from my shoulder blade, and mine arm be broken from the bone. - Job 31:16-22 (KJV)
Douay-Rheims inserts mercy as that which has been with him (Job), parenthetically, from his youth and mother's womb. While this fits the larger context of Job defending himself in his own eyes and before God, it is a bit of an interruption in the immediate context.
