Charles Ellicott Commentary Genesis 16:4

"And he went in unto Hagar, and she conceived: and when she saw that she had conceived, her mistress was despised in her eyes." — Genesis 16:4 (ASV)

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 although she would now hold the highest place in the household next to Sarai because of this relationship with Abram, she would nevertheless 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, in any case, acted as if she were Abram’s wife outright, and thrust Sarai aside.