Reference for Bekhorot 91:13

ובן תשעה שיצא ראשו מת

THESE ARE THE WORDS OF R'MEIR'BUT THE SAGES SAY: [IT IS NOT CONSIDERED AN OPENING OF THE WOMB] UNTIL [THE DISCHARGE] HAS THE FORM OF A HUMAN BEING.<span class="x" onmousemove="('comment',' And if not, the offspring which follows is a first-born also as regards redemption, for only the issue of the actual form of a human being is considered an opening of the womb exempting succeeding offspring from the law of redemption.');">⁹</span>

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