Last, she bore him a daughter, and named her Dinah.
The name Dina comes from the word din/judgment.
From here the Talmud derives that Leah passed judgment on herself and said: "Twelve tribes are destined to descend from Yakov, six came from me and four from the maidservants, which is a total of ten. If the fetus in me is a male, my sister Rachel will not even be equal to one the maidservants."
Immediately the fetus was transformed into a female, as it is stated: "And she called her name Dina". Meaning, she named her after passing her own judgment.
Another opinion holds that Dinah was Zevulun’s twin sister.
(Berachot 60a, Tanchuma 8, Yafe Tohar, Toledot Yitzchak, Ibn Ezra)
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