A woman’s menstrual cycle is often explained on a time scale of 28 days, which is the most common length of time between periods. However, some women experience regular variations (or infrequent fluctuations) ranging from as short as 20 days to as long as 40 days. Shorter and longer cycles are very normal and a woman is still fertile, they are just regarded as individual differences.
Some women find their cycles are consistently longer or shorter (eg. always around 35 days, or always around 24 days), while others notice fluctuations from month to month, or year to year and at different times of their lives. For example, 26 days one month and 31 days the next. Puberty and menopause are typical times to experience irregular periods, as the body goes through these major transitional phases.
Changes in period patterns can also happen as you grow older.
There are 5 phases in a normal menstrual cycle: