Zilpah gave birth to two sons, whom Leah claimed as her own and named Gad and Asher (Genesis 30:10–13).
According to the early rabbinical commentary Pirke De-Rabbi Eliezer, Zilpah and Bilhah, the handmaids of Leah and Rachel, respectively, were actually daughters of Laban and one or more of his concubines.
After Bilhah bore two sons, Leah took up the same idea and presented Zilpah to Jacob so she could have more children through her handmaid.
The morning after the wedding, Laban explained to Jacob, "This is not done in our place, to give the younger before the older" (Genesis 29:26).
In the novel The Red Tent by Anita Diamant, Zilpah and Bilhah are represented as half-sisters of Leah and Rachel by different mothers.