Objective: To estimate the unplanned pregnancy rate of calendar rhythm.
Design: Meta-analysis of eight studies of calendar rhythm published between 1940 and 1989.
Results: There exist few studies of the calendar rhythm method. Analysis of the best of these studies resulted in a conservative estimated Pearl rate of 18.5 +/- 1.8, and a less conservative estimate of 15.0 +/- 4.0, standardized to 12 months' observation; these results are in the range of other natural and barrier methods.
Discussion: We need properly done clinical trials of the calendar rhythm method to scientifically establish its effectiveness.