Halakhah for Niddah 70:13

And about this they, may their memory be blessed, said (Niddah 35b) that there is one 'wellspring' and the Torah made it impure, and the Torah [also] made it pure. [This is] meaning to say that the blood of birth that the Torah made impure and the blood of purification that the Torah made pure are from one place; and that is [that] both of them come out from the source (the uterus), which is a place of impurity. However the Torah made the matter depend on time and made impure the blood that comes from her at one time and made [it] pure at a second time. And blessed is the One that knows all wisdom, as He did every thing for a correct and necessary reason. And He, blessed be He, knows that up to a certain time, the blood is damaging; and [that] from then on, it is not damaging.