John Calvin 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 in her eyes. Here Moses relates that the punishment for excessive rashness quickly followed. The chief blame, indeed, rested with Sarai; yet because Abram had shown himself too trusting, God chastises both as they deserve.

Sarai is grievously and bitterly tried by the proud contempt of her servant; Abram is harassed by unjust complaints. Thus we see that both pay the penalty for their lack of seriousness, and that the plan devised by Sarai, and too eagerly embraced by Abram, fails to succeed.

Meanwhile, in Hagar, an instance of ingratitude is set before us, because she, having been treated with exceptional kindness and honor, begins to hold her mistress in contempt.

Since, however, this is an exceedingly common affliction of the mind, let the faithful accustom themselves to enduring it if, at any time, such an unjust return is made to them for their acts of kindness. But especially, let the weakness of Sarai move us to do so, since she was unable to bear the contempt of her servant.