Genesis 16:4

‘And he went in to Hagar and she conceived. And when she saw that she had conceived her mistress was of little account in her eyes.'

The plan was successful. But Hagar was an Egyptian and not brought up to tribal customs, and her success made her feel superior to her barren mistress. She sees herself as now the important wife and seizes the opportunity to take over that position. She begins to act in a superior way and to supplant her mistress as though her mistress were now of little importance. She does not accept her status as a producer of a child on Sarai's behalf.