Long cycles, late ovulation, and calendar rhythm.

Abstract

The basic relations between calendar rhythm and menstrual variation in a statistical orientation and in a perspective assessing requirements for a perfectly effective rhythm formula are reviewed. The empirical ju stifications of assumptions made about menstrual variation are also revi ewed. The normal distribution does not furnish a realistic model of cycle variation. The average cycle standard deviation of adult women is probably less than 3 days and the postovulatory phase is variable. The existence of occasional long cycles with late ovulation undermines any claim that calendar rhythm can provide absolute protection. Temperature rhythm to determine ovulation was designed to meet this problem of late ovulations. The effectiveness of calendar rhythm depends not only on the particular rhythm but also on the menstrual variabilites of the wome n using it.