When Rachel realized her sister Leah bore Jacob many children, while she bore none, gave Bilhah to Jacob, so she could have children through Bilhah, as that was a custom of that time and place. Bilhah bore two sons to Jacob, Dan, and Naphtali. Leah became jealous, so she gave her maidservant, Zilpah as a wife to Jacob. Zilpah bore two sons to Jacob, Gad and Asher.
Later on, Jacob's oldest son, who was born of a different woman, Reuben, committed adultery with Bilhah.