In the comments to this post, I was discussing Natural Family Planning with Feddie of Southern Appeal. I don't think much of NFP, if only because I see nothing wrong with conventional methods like surgical sterilization (now available without major abdominal surgery!) or hormones. But if you have a lot of time on your hands and don't mind either abstaining or using a back-up method for a third of the month or so (more like one half of your non-menstruating days), then an NFP method like Fertility Awareness might be an option.
All this talk about safe days and unsafe days got me thinking: Leviticus states that women are unclean for seven days following any bloody discharge. if you followed Levitical injunctions regarding sex, would you be more or less likely to conceive?
This is, like so many things, a matter of interpretation. If the initial determination is made on the first day of the woman's period, then she is clean again after seven days: more or less the length of the average woman's cycle. This allows for the maximum number of fertile clean days, since women are generally fertile during the middle of their cycle.
However, if the woman is unclean for seven days from the last day of her period, she will not be clean again until her fertile window has nearly passed.
So: Leviticus: baby maximizer or primitive birth control?