Abram's wife Sarai had not borne him children. She owned an Egyptian slave named Hagar.
References for Genesis 16:1
Sarai said to Abram, "Since the Lord has prevented me from bearing children, go to my slave; perhaps I can have children by her." And Abram agreed to what Sarai said.
References for Genesis 16:2
So Abram's wife Sarai took Hagar, her Egyptian slave, and gave her to her husband Abram as a wife for him. [This happened] after Abram had lived in the land of Canaan 10 years.
He slept with Hagar, and she became pregnant. When she realized that she was pregnant, she looked down on her mistress.
References for Genesis 16:4
Then Sarai said to Abram, "You are responsible for my suffering! I put my slave in your arms, and ever since she saw that she was pregnant, she has looked down on me. May the Lord judge between me and you."
References for Genesis 16:5
Abram replied to Sarai, "Here, your slave is in your hands; do whatever you want with her." Then Sarai mistreated her so much that she ran away from her.
References for Genesis 16:6