Genesis 16:4

For barrenness in itself 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 privilege 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.