Reference for Bekhorot 69:48

מאי טעמא נעקרו הוי מומא משום דכי פעי מיחזי:

Said Rab Judah: It is permitted to cause a blemish to a firstling before it is born.<span class="x" onmousemove="('comment',' Lit., 'came-forth into the lighted space of the world'. The reason is because a first-born is only hallowed after leaving the womb.');">²³</span>

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