EXTRA · Parashas Vayeishev · Passuk 38:27 · not a Torah Temimah citation on this passuk
When the time came for her to give birth, there were twins in her womb!
Classical source (EXTRA side-ring): Bereshis Rabbah 85:13.
Sefaria — Bereshis Rabbah 85:13
English: “And it was at the time of her giving birth, and behold, twins” — here [the spelling is] full; elsewhere deficient: for Peretz and Zerach were both righteous.
Derash on the verse (Bereshis Rabbah) — not a formal named middah of the 13. Orthography is tied to the character of the children.
The full writing of “twins” signals that both Peretz and Zerach stood in righteousness.