Musar for Niddah 70:39

When a woman experiences the pangs of birth, this results in impurity. After the birth, the impurity has been released through her pain. After the seven days of recovery from such impurity she no longer contains the ingredients that have caused it and therefore is free from any new impurity for a period longer than normal. Since, in the case of the birth of a female, the original impurity was stronger and lasted longer, the period during which she will be free from again contracting such impurity is correspondingly longer. [I have paraphrased the author's quotation of th anonymous source. Ed.] Another reason for that is that man lost his garments of light due to his sin and had to wear garments made of skin, i.e. something removed from the flesh of some animal. He became susceptible to נגע צרעת, afflictions of his skin. The Torah (Leviticus 13,1 expresses this by saying: נגע צרעת כי תהיה באדם … אדם כי יהיה בו נגע צרעת בעור בשרו. What all this means is that Adam, who was the cause of man having to wear clothing made of skin, is the cause of these skin diseases which afflict those who engage in slander. The serpent had been the one to indulge in slander against the Lord Himself when it said that Eve could safely eat from the tree of knowledge (Genesis 3,4) without incurring mortality. It seduced Eve and Adam to violate G–d's command.