הלכה על נדה 50:17

And we have already said in the warning of the 'torn' animal (Sefer HaChinukh 73) and in the prohibitions of foods (Sefer HaChinukh 147, 148) that God, blessed be He, distanced His people that He chose from all the things that hurt the body, in that it is a vessel for the soul that knows its Creator. And so too from this root did He distance us from the woman who has given birth, from the menstruant and from the zavah (a woman with an irregular discharge) for a few days, until they become cleansed from that excess that they have, which is a bad and infectious thing. Therefore they, may their memory be blessed, said (Rashi on Leviticus 12:2) that a woman never [experiences] blood unless her head and limbs are heavy upon her. And there is no doubt that this excess makes them, and anyone who comes close to them at all, sick. And all the more so with sexual relations, which is the epitome of physical closeness. And also the offspring that is born [from a union] while that excess overpowers her is destined for bad illnesses. And from this root that we said from the angle of excess, the Torah commanded that she be impure from the male seven [days] and two weeks from the female. As there is no doubt that the excesses of the woman that gives birth to a female are greater than one who gives birth to a male - since conception of a male indicates heat in the female. And [it is] like they, may their memory be blessed, said (Berakhot 60a), "'If a woman conceives (literally, gives seed) and gives birth to a male - [if] a woman gives seed first, she gives birth to a male." And it is well-known that the excesses of heat are few and its action is always quick - it is a logical thing. And therefore seven days of cleansing suffices with the pregnancy of a male.