Genesis 16:4 - Ellicott's Commentary On The Whole Bible

Her mistress was despised. — Hagar, we are told in Genesis 16:3, was to be, not Abram’s concubine, but his wife. She was to be Sarai’s representative, and though now she would hold the highest place in the household next to Sarai, because of this relation to Abram, yet she would continue to be Sarai’s maid. But no sooner had she conceived, than, proud of her superiority over her mistress, she wished to overthrow this arrangement, and, at all events, acted as if she was Abram’s wife absolutely, and thrust Sarai aside.