Zilpah, Leah"s maid, bore Jacob two sons: Gad and Asher. [source][source][source]
"The terms wife and concubine are used more loosely in the patriarchal period. Three women in the patriarchal period are called both wife and concubine: Hagar ( Genesis 16:3; Genesis 25:6 indirectly), Keturah ( Genesis 25:1; cf. Genesis 25:6; 1 Chronicles 1:32), and Bilhah ( Genesis 30:4; Genesis 35:22). Each of these concubines is an auxiliary wife to the patriarch, not a slave, but subordinate to the wife who is her mistress. After the patriarchal period, the term wife is never used as a synonym for concubine. Zilpah, though never called a concubine (cf. Genesis 30:9), has the same social position as Bilhah (cf. Genesis 37:2)." [1][source]