Reference for Ketubot 181:12

אבל אחת בחייו ואחת במותו והא דלאחר מיתה יולדת זכר היא אימא ליחוש לאינצויי קמ"ל:

but [it does not apply to a case where] one [wife died] during her husband’s lifetime and the other after his death and the latter had given birth to a son, [since in this case] we would have to be concerned about quarreling. Therefore, he teaches us [that even if one dies during her husband’s lifetime and one after his death, the sons of the first receive their ketubah].

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