Commentary for Genesis 16:1

שפחה מצרית A HANDMAID, AN EGYPTIAN — She was a daughter of Pharaoh; when he saw the miracles which had been performed for Sarah’s take he said, “It is better for my daughter to be a handmaid in this man’s house than be mistress in another man’s house” (Genesis Rabbah 45:1).

ושרי אשת אברם לא ילדה לו, as mentioned already in Genesis 11,30 ותהי שרי עקרה , "Sarai was barren." Now when Sarai realized that her husband was already 85 years old, and she still had not been able to bear a child for him, while she herself had already reached the age of 75, she thought that she had no longer any hope of conceiving herself. She therefore reasoned to herself, that seeing G'd had promised Avram that he would have children of his own who would inherit the land of Canaan, G'd must have referred to his siring children from another woman. She reasoned further that it would be in her own best interest that any children born to her husband should be born by a woman under her control so that she would experience the joy of motherhood at least vicariously.