Halakhah for Niddah 62:28
אמר להן
And why did the Torah ordain that in the case of a male [the woman is clean] after seven days and in that of a female after fourteen days? [On the birth of a] male with whom all rejoice she regrets her oath after seven days, [but on the birth of a female] about whom everybody is upset she regrets her oath after fourteen days. And why did the Torah ordain circumcision on the eighth day?<span class="x" onmousemove="('comment',' After birth, and not on the seventh which is the last day of uncleanness ');"><sup>27</sup></span>
Sefer HaChinukh
It is from the roots of the commandment [that it is] in order that she be aroused by action to give thanks to God, blessed be He, who saved her from the pangs of her birth, which is a miraculous thing. And also they, may their memory be blessed, said (Niddah 31b) that a woman rushes to swear at the time of her throes that she will no longer [have sexual relations with] a man. And therefore she requires atonement, and hence her sacrifice is called a sin-offering, and it is stated about her, "and the priest shall atone for her" (Leviticus 12:8).
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