Reference for Niddah 62:4

אמר רבא

further stated: A woman conceives only immediately before her menstrual period, for it is said, Behold I was brought forth in iniquity;<span class="x" onmousemove="('comment',' Ps. LI, 7. The last word is taken as an allusion to the menstruation period when intercourse is an iniquity' and the prefixed beth ('in') is rendered 'near'. ');">³</span>

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