Menstrual cycle roughly corresponds in length with the lunar cycle, which is one of the foundations of most calendars. The word "menstruation" comes from Latin "mensis" which means "month" - the relationship was always obvious.

However, no actual causal link between them is known to exist. The cycles are not quite the same length anyway - 28 days for menstruation, 29.53 for the lunar cycle. Despite this, many theories have been proposed which try to relate the two - for example, by saying that the menstrual cycle is influenced by the varying amount of light during night time. None of these theories are accepted. Many non-human mammals have completely different menstrual cycle lengths, which also suggests that the length of 28 days is simply a coincidence.