All menstrual cycles last 28 days.

Myth: a woman’s menstrual cycle can range anywhere from 21 days to 35 days depending on her age and other various health factors.

The menstrual cycle, which is counted from the first day of one period to the first day of the next, isn’t the same for every woman. Menstrual flow might occur every 21 to 35 days and last two to seven days. For the first few years after menstruation begins, long cycles are common. However, menstrual cycles tend to shorten and become more regular as you age.

Your menstrual cycle might be regular — about the same length every month — or somewhat irregular, and your period might be light or heavy, painful or pain-free, long or short, and still be considered normal. Within a broad range, “normal” is what’s normal for you.

Tracking your menstrual cycles can help you understand what’s normal for you, time ovulation, and identify important changes — such as a missed period or unpredictable menstrual bleeding. While menstrual cycle irregularities usually aren’t serious, sometimes they can signal health problems.

If you’re concerned about your periods, then also make note of the following every month:

• End date. How long does your period typically last? Is it longer or shorter than usual?
• Flow. Record the heaviness of your flow. Does it seem lighter or heavier than usual? How often do you need to change your sanitary protection? Have you passed any blood clots?
• Abnormal bleeding. Are you bleeding in between periods?
• Pain. Describe any pain associated with your period. Does the pain feel worse than usual?
• Other changes. Have you experienced any changes in mood or behavior? Did anything new happen around the time of change in your periods?

Menstrual cycle irregularities can have many different causes, including:

• Pregnancy or breastfeeding
• Eating disorders, extreme weight loss, or excessive exercising
• Polycystic ovary syndrome (PCOS)
• Premature ovarian failure
• Pelvic inflammatory disease (PID)
• Uterine fibroids