Every woman's menstrual cycle is different from other women. Your period can last longer / shorter durations of time, the flow can vary, the gap between cycles can be different too.
Keeping track of your cycle, helps you to be better prepared. The lesser are your chances of being caught unawares.
As hormone levels in your body fluctuates through the menstrual cycle, it causes changes to your body including
The chart you maintain will help you not just predict the timing of your period with greater accuracy but also note the changes in your body. This helps you better understand and deal with the pre-menstrual symptoms, with gauging the times of ovulation (incase you're trying to have a baby) etc.
Also, incase you have to visit your doctor, it helps you to be better prepared with details of your menstrual cycle, fix appointments for tests like pap smears which must not be done while you have your period.
Maintaining a calendar helps you make better decisions regarding your reproductive health.
Easy Prediction of Your Period Cycle
Learn more about Period Calendar