Zilpah is the foremother of two of the tribes of Israel, Gad and Asher. Though she occupies a lauded position in the lineage of Israel, Zilpah's role within Jacob’s household is complex. She is introduced as the slave of Jacob’s wife, Leah. Leah, in competition with her sister Rachel (who’s also married to Jacob), gives Zilpah to Jacob to impregnate. Zilpah births two sons, but it’s Leah who chooses the babies' names. Leah’s namegiving is telling; Gad and Asher were considered Leah’s children. Zilpah may have been given to Jacob “as a wife,” but her body is treated as a vessel in the reproductive contest between the two sisters (Genesis 30:9). That her life, and her children’s lives, aren’t as valuable to Jacob as the lives of Leah and Rachel is made clear in Genesis 33. In that story, Jacob fears a violent attack by Esau, so he lines up his family in order of preference.
Zilpah (and her counterpart Bilhah) are placed in the most vulnerable position. Though Zilpah occupies a lower position in Jacob’s household, her children are of equal status with the biological children of her mistress: Gad and Asher become two of the twelve tribes of Israel. “Through the wombs of Rachel, Leah, Bilhah, and Zilpah, Israel’s people were birthed by choice and by force” (Wilda C. Gafney, Womanist Midrash, 112).
A full ⅓ of the tribes of Israel come from the enslaved women Bilhah and Zilpah.
In the family system depicted in the patriarchal narratives, “a woman’s biology is part of her destiny…a woman establishes her worth as a wife through her ability to increase her husband’s lineage; her prestige is determined by her reproductive capabilities” (Naomi Steinberg, Women in Scripture, 170).
Zilpah is the grandmother of Serah. Serah is mentioned only briefly in Scripture, but is depicted as beautiful, wise, and extraordinarily long-lived in classical midrashic tradition. You can read about Serah in Tamar Kadari’s article, “Serah, daughter of Asher: Midrash and Aggadah.”