Anna has a post on the Jewish family purity laws, and the discipline of following them.
Now if you add up the days, it sounds a bit like Natural Family Planning but with the plan being maximize your chances for getting pregnant. Usually I would just read through this part of the Old Testament, see 'unclean' and 'blood', and figure it was about that. Indeed, Anna doesn't mention my point either.
But I'm a bit sensitive to this because I believe God really wants us to remember that He is the God of the living.
As Christians, even as Catholics (despite the stereotype) I think we miss that point too often. How well do you weight having children in your moral calculus? Is there always something just a bit more important to do? Can't you see how the enemy, wanting to thwart God, could feed you with big world-changing ideas, even apparently Christian ideas that mysteriously necessitate continued childlessness?