Even though the word “menstruation” comes from the same Latin root as “moon” and “month,” menstrual periods are rarely as regular and predictable as lunar cycles. In fact, it’s very common for them to change throughout your reproductive years. They’re a reflection of your physical and emotional health and your hormonal shifts. So the timing, flow and duration you have in your teens and 20s can alter in your 30s, then morph again in your 40s. You might find yourself suddenly cramp-free, or puzzled at irregular cycles, or worried when you need super-absorbent pads.

From puberty to menopause, you will probably spend the equivalent of about five entire years dealing with your period, so it’s important to know which changes are normal (most are) and which should prompt a talk with a health professional.