on calendar rhythm. CycleBeadsare a method of determining potentially fertile days based on thousands of cycles examined through the Ovulation Method (Billings) and then turning that data into a simple algorithm. This method only works for women with regular cycles that are 26 to 32 days in length (Basically, the same group for whom calendar rhythm is effective). What is interesting is that this method is as effective at pregnancy prevention as most other user-dependant methods.
Rhythm has actually gotten an undeserved bad name. Calendar rhythm is 80% plus effective at pregnancy prevention when properly understood and utilized. The real problem with calendar methods is that they are not effective for pregnancy prevention immediately post-partum (although breastfeeding can be) or during menarche and menopause related cycle irregularities.
Addendum: Amy Welborn has a couple of related posts on the topic.
see here(warning;148 c0mments at last sighting) and here
So does Charles De Nunzio
And an article about the moral dangers of some attitudes here
