The menstrual cycle is a natural biological process that prepares the body for a possible pregnancy each month. It is a normal part of reproductive health for many women, girls and people who menstruate. While the average menstrual cycle lasts 28 days, cycles ranging from 21 to 35 days in adults and 21 to 45 days in adolescents during the first few years after menarche are generally considered normal.
The menstrual cycle is regulated by hormones produced by the brain and ovaries. These hormones work together to prepare the uterus for pregnancy. If pregnancy does not occur, the lining of the uterus is shed through menstruation, commonly known as a period.
Understanding the menstrual cycle helps individuals recognise what is normal for their bodies, identify changes that may require medical attention and make informed decisions about their health and wellbeing.
The Four Phases of the Menstrual Cycle
- Menstrual Phase
The menstrual phase begins on the first day of menstrual bleeding and usually lasts between 2 and 7 days, although this varies from person to person.
During this phase:
• The lining of the uterus (endometrium) is shed.
• Blood, mucus and uterine tissue leave the body through the vagina.
• Hormone levels, particularly oestrogen and progesterone, are at their lowest.
Common experiences during menstruation include:
• Mild abdominal cramps
• Lower back discomfort
• Fatigue
• Breast tenderness
• Mood changes
• Headaches
These symptoms vary widely between individuals.
- Follicular Phase
The follicular phase begins on the first day of menstruation and continues until ovulation.
During this phase:
• The brain releases follicle-stimulating hormone (FSH).
• Several ovarian follicles begin to develop.
• Usually one follicle becomes dominant and matures.
• Oestrogen levels gradually increase.
• The uterine lining begins to rebuild after menstruation.
Many people notice:
• Increased energy
• Improved mood
• Better concentration
- Ovulation
Ovulation usually occurs around the middle of the menstrual cycle, although the exact timing differs between individuals and cycles.
During ovulation:
• A surge in luteinising hormone (LH) causes the ovary to release a mature egg.
• The egg travels into the fallopian tube.
• Pregnancy can occur if sperm fertilises the egg.
Some individuals experience:
• Mild pain on one side of the lower abdomen (sometimes called mittelschmerz)
• Increased cervical mucus that becomes clear and slippery
• Increased libido
Ovulation is the most fertile period of the menstrual cycle.
- Luteal Phase
After ovulation, the follicle that released the egg becomes the corpus luteum, which produces progesterone.
Progesterone helps:
• Maintain the uterine lining
• Prepare the uterus for implantation if pregnancy occurs
If pregnancy does not occur:
• Progesterone and oestrogen levels decrease.
• The uterine lining breaks down.
• A new menstrual period begins.
Common symptoms during the luteal phase may include:
• Bloating
• Mood changes
• Food cravings
• Acne
• Breast tenderness
• Fatigue
Hormones Involved in the Menstrual Cycle
Four hormones play central roles in regulating the menstrual cycle:
Hormone Main Function
Follicle-stimulating hormone (FSH) Stimulates ovarian follicles to develop.
Luteinising hormone (LH) Triggers ovulation.
Oestrogen Rebuilds the uterine lining and supports follicle development.
Progesterone Maintains the uterine lining after ovulation.
These hormones fluctuate naturally throughout the menstrual cycle.
What Is Considered Normal?
Menstrual cycles vary between individuals. A healthy menstrual cycle may include:
• Cycle length between 21 and 35 days in adults
• Menstrual bleeding lasting 2–7 days
• Mild discomfort that does not interfere significantly with daily activities
• Some variation from month to month
Adolescents may experience irregular cycles during the first few years after their first period because hormone patterns are still maturing.
When Should You Seek Medical Advice?
Consult a qualified healthcare provider if you experience:
• Periods that suddenly stop (when not pregnant)
• Very heavy bleeding requiring frequent pad or tampon changes
• Bleeding lasting longer than seven days
• Severe pain that prevents normal daily activities
• Bleeding between periods
• Menstrual cycles that remain consistently shorter than 21 days or longer than 35 days in adults
• Menstruation that has not started by age 15, or within three years after breast development begins
Early assessment can help identify underlying conditions such as endometriosis, polycystic ovary syndrome (PCOS), thyroid disorders or bleeding disorders.
Frequently Asked Questions
Is a 28-day cycle normal?
Yes. A 28-day cycle is common, but cycles between 21 and 35 days in adults are also considered normal.
Is it normal for periods to be irregular during adolescence?
Yes. During the first few years after menarche, cycles are often irregular while the reproductive hormone system matures.
Can stress affect the menstrual cycle?
Yes. Physical illness, significant emotional stress, changes in body weight, intense physical activity and some medications can influence the menstrual cycle.
Can I exercise during my period?
Yes. Most people can continue their usual physical activity during menstruation. Exercise may even help reduce menstrual cramps for some individuals.
Key Takeaways
• The menstrual cycle is a normal biological process.
• Most adult menstrual cycles last between 21 and 35 days.
• The cycle consists of four phases: menstruation, follicular phase, ovulation and luteal phase.
• Hormones regulate each stage of the cycle.
• Understanding your menstrual cycle can help you recognise what is normal and identify changes that may require medical attention.
References
1. World Health Organization. Adolescent and young adult health.
2. International Federation of Gynecology and Obstetrics (FIGO). Menstrual Health and Hygiene Guidance.
3. American College of Obstetricians and Gynecologists (ACOG). Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign.
4. UNICEF. Guidance on Menstrual Health and Hygiene.
5. International Federation of Gynecology and Obstetrics (FIGO). Good Practice Recommendations on Menstrual Health.