John Gill 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)

And he went in to Hagar, and she conceived
The formality of the marriage being over, he enjoyed her as his wife, and she immediately conceived by him:

and when she saw that she had conceived ;
when she perceived that she was with child:

her mistress was despised in her eyes ;
she thought herself above her, and treated her as her inferior, with contempt, and reproached her for her barrenness, as Peninnah did Hannah, (1 Samuel 1:6 1 Samuel 1:7) ; and it was the more ungrateful, as it was at the motion of her mistress that she was given to Abram for wife.