Reference for Niddah 58:29

שבוע שלישי מטבילין לה ביממא אימר

every day might be regarded as being possibly the conclusion of [the clean days prescribed for] a childbirth and the beginning of the period of menstruation, so that the twenty-eighth day itself<span class="x" onmousemove="('comment',' The last day of the fourth week. ');">²⁷</span>

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