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In the beginning of parshas כי תצא (Devarim 21:15-17), the Torah commands that in a case of one man having two wives who both bear him sons - that the first-born, even if it is from the wife he loves less, should be given the בכורה.

כִּי־תִהְיֶיןָ לְאִישׁ שְׁתֵּי נָשִׁים הָאַחַת אֲהוּבָה וְהָאַחַת שְׂנוּאָה וְיָלְדוּ־לוֹ בָנִים הָאֲהוּבָה וְהַשְּׂנוּאָה וְהָיָה הַבֵּן הַבְּכֹר לַשְּׂנִיאָה׃ וְהָיָה בְּיוֹם הַנְחִילוֹ אֶת־בָּנָיו אֵת אֲשֶׁר־יִהְיֶה לוֹ לֹא יוּכַל לְבַכֵּר אֶת־בֶּן־הָאֲהוּבָה עַל־פְּנֵי בֶן־הַשְּׂנוּאָה הַבְּכֹר׃ כִּי אֶת־הַבְּכֹר בֶּן־הַשְּׂנוּאָה יַכִּיר לָתֶת לוֹ פִּי שְׁנַיִם בְּכֹל אֲשֶׁר־יִמָּצֵא לוֹ כִּי־הוּא רֵאשִׁית אֹנוֹ לוֹ מִשְׁפַּט הַבְּכֹרָה׃

But in a case where the two wives are both pregnant at the same time, and the wife with whom a child was conceived second gives birth first - which son gets the בכורה; the one conceived first or the one born first?

I vaguely remember hearing (perhaps in the name of the גר"א?) that we learn from the words כי הוא ראשית אונו that the first-conceived child is the בכור even if he ends up being born second. I looked through both the Mishneh Torah hilchos nachalos, and the Shulchan Aruch choshen mishpat siman 277, but couldn't find any explicit psak in regard to this.

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    Also relevant with one wife in cases of IVF
    – Heshy
    Commented Nov 15, 2023 at 13:03
  • @Heshy Or fraternal twins
    – Double AA
    Commented Nov 15, 2023 at 14:44
  • @DoubleAA Is there a way of telling which of two fraternal twins was conceived first?
    – Joel K
    Commented Nov 15, 2023 at 14:50
  • @JoelK Not that I know of, but then it's a safek and neither claims bechora. Same applies to most ivf cases, where multiple eggs are fertilized at a time after retrieval. Plus ראשית אונו doesn't mean conception really but something more like which particular sperm cell exited first.
    – Double AA
    Commented Nov 15, 2023 at 15:03

1 Answer 1

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Netziv in Ha'amek Davar to Devarim 21:15 points out that we follow the time of birth, not conception, and shows how this is derived from the verses here.

He also notes, parenthetically, a contrary tradition from Gra (that we would follow the time of conception) but writes that he doesn't believe it possible that Gra said such a thing.

(Also relevant is Rashi to Bereishit 25:26 which implies (in a much more midrashic context) that the first conceived has some type of claim to be the firstborn.)

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