Reference for Niddah 58:30
יולדת נקבה בזוב היא בלילותא ב"ש לטעמייהו דאמרי
might be presumed to be the first day of the menstrual period and she must consequently continue [her uncleanness for] seven days in respect of her menstruation.<span class="x" onmousemove="('comment',' Which, beginning on the last day of the fourth week, terminates on the sixth day of the fifth week. Hence the permissibility of marital duty (after due ritual immersion) on the night following that day (the one preceding the thirty-fifth day of her return). During the weeks that follow all intercourse would be forbidden, since each alternate 'clean' week might he regarded as the period of seven days that must be allowed to elapse after the zibah of the previous 'unclean' week before cleanness is attained. ');"><sup>28</sup></span>