What is menstruation?
Written by Dr. Specialist Stine Andreassen
Specialist in Gynecology and Obstetrics, PhD in Obstetrics in 2015 and more than 15 years of clinical practice in Obstetrics and Gynecology
Menstruation, often called a period, is the monthly shedding of the uterine lining when pregnancy does not occur. It is a normal part of the reproductive cycle and is regulated by hormones that prepare the body for a possible pregnancy each month.
While every menstrual cycle is different, understanding how menstruation works can help you recognize what is normal for your body and when it may be helpful to seek medical advice.
Why do we have periods?
Menstruation occurs as part of the body's reproductive cycle. Each month, hormones stimulate the ovaries to prepare an egg and the uterus to build a thick lining called the endometrium. This lining is designed to support a pregnancy if fertilization occurs. If pregnancy does not happen, hormone levels fall and the uterine lining sheds. This tissue leaves the body through the vagina and is known as a menstrual period. Periods are a sign that the reproductive system is responding to hormonal changes, although not all menstrual cycles result in ovulation.
Read more: What is ovulation?
What is considered a normal period?
A normal menstrual cycle can vary significantly from person to person. But in general:
A menstrual cycle typically lasts between 21 and 35 days
Bleeding usually lasts between 2 and 7 days
Menstrual flow may be light, moderate, or heavy
Mild cramping is common
Your usual pattern is often more important than comparing your cycle to someone else's.
What symptoms can occur during a period?
Many people experience symptoms before or during menstruation. Common symptoms include:
Menstrual cramps
Bloating
Breast tenderness
Headaches
Fatigue
Mood changes
Food cravings
Symptoms can vary from month to month and throughout different stages of life.
When should my first period start?
The first menstrual period is called menarche. Most people have their first period between the ages of 10 and 15 years. The timing varies and may be influenced by:
Genetics
Nutrition
Body weight
Physical activity
General health
Most people begin menstruating within a few years after breast development starts.
What if I have not started my period?
A delayed first period is called primary amenorrhea. Possible causes include:
Normal variation in puberty timing
Low body weight
Nutritional deficiencies
Hormonal imbalances
Certain genetic conditions
Structural differences in the reproductive system
Medical evaluation is often recommended if menstruation has not started by age 15 or if puberty appears delayed.
Why has my period stopped?
Missing periods after menstruation has already started is called secondary amenorrhea. Pregnancy is one of the most common reasons for a missed period, but many other factors can affect the menstrual cycle. Possible causes include:
Stress
Significant weight loss
PCOS (Polycystic Ovary Syndrome, also referred to as Polycystic Ovarian Morphology Syndrome (PMOS)
Thyroid disorders
Certain medications
Chronic illness
Perimenopause
Menopause
Occasional cycle changes are common, but persistent absence of periods should be discussed with a healthcare professional.
Why are my periods irregular?
Irregular periods are common, particularly during adolescence and the years leading up to menopause. An irregular cycle may mean:
Periods occur at unpredictable times
Cycle length varies significantly
Periods are occasionally skipped
Possible causes include:
Normal hormonal fluctuations
PCOS
Thyroid disorders
Stress
Significant weight changes
Perimenopause
Tracking your menstrual cycle can help identify patterns and provide useful information for healthcare professionals.
Why is my period so heavy?
Heavy menstrual bleeding can affect quality of life and may sometimes lead to iron deficiency or anemia. Potential causes include:
Hormonal imbalances
Fibroids
Adenomyosis
Bleeding disorders
Endometrial polyps
Certain medications
A healthcare professional can help determine the cause and discuss treatment options.
Why are my periods so painful?
Painful periods are known as dysmenorrhea. Menstrual cramps occur when the uterus contracts to shed its lining. Mild discomfort is common, but severe pain is not something you simply have to accept.
Possible causes include:
Primary dysmenorrhea (common menstrual cramps)
Endometriosis
Adenomyosis
Fibroids
If period pain regularly affects school, work, exercise, sleep, or daily activities, it may be helpful to seek medical advice.
Related resource: What is endometriosis?
What is premenstrual syndrome (PMS)?
Premenstrual syndrome (PMS) refers to a group of physical and emotional symptoms that occur before menstruation. Common symptoms include:
Mood swings
Irritability
Anxiety
Bloating
Breast tenderness
Fatigue
For most people, symptoms improve once menstruation begins.
How do periods change throughout life?
Menstrual cycles naturally change over time.
During adolescence: Periods are often irregular during the first few years after menstruation begins.
During the reproductive years: Cycles usually become more predictable and regular.
During pregnancy: Periods stop during pregnancy.
During perimenopause: Cycles often become more irregular as hormone levels fluctuate.
During menopause: Menstruation permanently stops after 12 consecutive months without a period.
When should I see a healthcare professional?
Consider seeking medical advice if you:
Have not started menstruating by age 15
Suddenly stop having periods
Experience very heavy bleeding
Have severe menstrual pain
Bleed between periods
Notice major changes in your usual cycle
Have concerns about fertility or reproductive health
Early evaluation can help identify underlying causes and provide appropriate treatment if needed.
Related resources
What is PCOS?
What are early pregnancy symptoms?
Learn more about fertility
FAQ – Menstruation
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Menstruation is the monthly shedding of the uterine lining when pregnancy does not occur.
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Most periods last between 2 and 7 days.
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A normal menstrual cycle typically occurs every 21 to 35 days.
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Irregular periods may be caused by hormonal changes, stress, PCOS, thyroid disorders, weight changes, or perimenopause.
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Period pain occurs when the uterus contracts to shed its lining. Severe pain may be associated with conditions such as endometriosis or adenomyosis.
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Medical advice may be helpful if you experience severe pain, heavy bleeding, missed periods, bleeding between periods, or significant changes in your menstrual cycle.