This paper critically examines Weller and Weller's preferred last month only method for measuring synchrony. Within-woman and between-women menstrual-cycle variability are distinguished. If there is within-woman cycle variability, synchrony requires a process of entrainment. Between-women cycle variability precludes synchrony between rhythms that are not integer multiples of each other. The assumptions of Weller and Weller's last month only measurement model are tested by computer simulation under conditions of cycle variability. It is demonstrated that these assumptions are biased towards finding synchrony when it does not exist and that the degree of error is an increasing function of cycle variability. Indeed, the error uncovered quantitatively predicts the peculiar skew in their data distributions. Synchrony is almost impossible when there is cycle variability and finding synchrony may be an indicator of a methodological artifact rather than a phenomenon.