Rashi on Genesis 38:27/רש"י על בראשית ל״ח:כ״ז

1בעת לדתה AT THE TIME OF HER BEARING — But in the case of Rebecca Scripture says (25:24) “And when her days to give birth were fulfilled” — in the latter case the months of pregnancy were complete, here, however, they were short of the full term (Genesis Rabbah 85:13).

2והנה תאומים AND BEHOLD TWINS — Here the word is written plene (with א and י whilst there (in the case of Rebecca) it is written defective (תומם without these letters) because one (viz., Esau) was wicked, but here both were righteous (Genesis Rabbah 85:13).