Rashi on Genesis 16:4/רש"י על בראשית ט״ז:ד׳

1ויבא אל הגר ותהר AND HE CAME UNTO HAGAR AND SHE CONCEIVED from the first union (Genesis Rabbah 45:4).

2ותקל גברתה בעיניה HER MISTRESS WAS SLIGHTED IN HER EYES — She said, “As regards this woman Sarai, her conduct in private can certainly not be like that in public: she pretends to be a righteous woman, but she cannot really be righteous since all these years she has not been privileged to have children, whilst I have had that blessing from the first union” (Genesis Rabbah 45:4).