This is one of the shorter chapters in the Bible. The note needs to be made that the uncleanness was not from the birth itself, but from the bleeding that went along with it. On a medical note, this blood, as with any blood, was and is a rich source of nourishment for every bacteria and parasite on the block. Keeping the woman isolated after birth protected her, the baby, and the community from any possible cross-contamination. Why was the time twice as long after the birth of a girl as of a boy? There is no known historical or theological or scientific reason. But there may be a reason in an entirely different area: behavior. Little girls, from birth on, are far more fascinated by human faces than other things. Little boys like bright colored objects, especially if they move. Thus, a little girl might benefit from staying more or less alone with her mother for almost three months, while a little boy would be quite happy checking out a moving and colorful outside world in a little more than a month. God thinks of everything. Maybe this was the sort of thing He was thinking when the rules regarding purification after childbirth were given to the Israelites. Then, again, maybe that's not it at all. The one thing we can be sure of is that HE had a reason, and a good one, whether or not we will ever know what it was.