Reference for Bekhorot 42:21

בבהמה בכמה

Said R'Hisda: Behold the Sages said: The period for the formation of an embryo in a woman is forty days.<span class="x" onmousemove="('comment',' And should the woman, therefore, have a miscarriage before this period has elapsed. she is not required to keep the days of purity and impurity laid down by Scripture for a woman after childbirth.');">¹⁵</span>

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