Matthew Poole and continuators on Genesis 16:4

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 .

For barrenness in it self was a reproach, and especially to Sarai, who seemed to be a person rejected by God, as one whom he would not honour with being the mother of that seed: and Hagar being suddenly made Sarai's partner in the priviledge of Abram's bed, and superior to her in respect of that great blessing of child-bearing, it is no wonder if she grew insolent upon it, especially being advanced so highly from so low a condition.