The menstrual clock, an open access tool to identify and illustrate menstrual phases, windows, and events

The menstrual cycle is a biological rhythm, the effects of which are challenging to investigate due to important variability in cycle characteristics, including cycle duration, both between and within individuals. Here, we introduce a tool that calculates this variability, identifies menstrual phases and even more granular menstrual windows, and enables an efficient illustration of results on a circular plot, or menstrual clock. A degree angle approach is used, graduating the follicular phase (menses to ovulation) from 0° to 180° and the luteal phase (ovulation to next menses) from 180° to 360°. This approach enables comparisons between cycles of different duration and the visualization of the distribution of events (symptoms or treatment) and variation of levels (temperature, metabolites, secondary effects, etc.) across the cycle. The open access MenstrualClock R package, incorporating this method in four functions, has been created to facilitate its implementation in research. By calculating menstrual cycle angles, the menstrual clock method provides a convenient tool that can be used to standardize and visualize menstrual cycle phase. Future studies can test the tool against hormone profiles and develop it further to advance research about menstrual cycle effects.