The Menstrual Cycle: A Complete Guide

Your Menstrual Cycle Is More Than Your Period

For many people, the menstrual cycle begins and ends with a period. We mark the first day of bleeding on a calendar, count the days until the next one, and may wonder why the timing, pain, mood or energy levels seem different from month to month.

But the menstrual cycle is much more than the days you bleed.

It is a coordinated process involving the brain, hormones, ovaries and uterus. These systems communicate with one another to support the development of an egg, the release of that egg through ovulation, and changes in the uterine lining that prepare the body for a possible pregnancy. If pregnancy does not occur, hormone levels change and the uterine lining is shed as a period. 1

Understanding this process can help you recognise your usual pattern, understand what your body may be signalling and identify when a change deserves medical attention.

The goal is not to make every cycle fit a perfect calendar. It is to understand what is normal for you while knowing when to seek help.

What Is the Menstrual Cycle?

The menstrual cycle is the series of hormonal and physical changes that prepare the reproductive system for a possible pregnancy.

Day 1 of your menstrual cycle is the first day of menstrual bleeding. The cycle continues until the day before your next period begins. The first day of the next period marks the start of a new cycle. 2

For example, if one period begins on 1 October and the next begins on 29 October, the cycle length is 28 days.

A 28-day cycle is often used as an example in textbooks and apps, but it is not a universal standard. Cycle length varies between individuals and can change over time.

For adults, FIGO describes a typical menstrual-cycle frequency range of 24–38 days, while ACOG patient guidance commonly uses 21–35 days. These ranges come from different clinical frameworks. A single cycle length cannot, by itself, diagnose a menstrual disorder. Age, symptoms and changes from your usual pattern also matter. 2 3

What matters is your usual pattern, how much it changes, your age and whether you have symptoms.

The Four Main Stages of the Menstrual Cycle

The cycle is often described in four stages: menstruation, the follicular phase, ovulation and the luteal phase. These stages are connected, and their timing differs from one person and cycle to another.

1. Menstruation: When Your Period Begins

Menstruation is the shedding of the uterine lining through the vagina. The bleeding contains blood and tissue from the lining of the uterus.

At the end of the previous cycle, if pregnancy has not occurred, levels of oestrogen and progesterone fall. This hormonal change triggers the uterine lining to break down and shed. 1

A period commonly lasts around 2–7 days, although bleeding patterns vary. Flow may be heavier during the first few days and become lighter towards the end. 2

You may experience cramps, fatigue, headaches, changes in bowel habits or mood symptoms around this time. Some discomfort can occur, but severe pain that interferes with daily life should not be dismissed as something you simply have to endure.

2. The Follicular Phase: Preparing an Egg

The follicular phase begins on the first day of your period and continues until ovulation. This means it overlaps with menstruation.

The brain releases follicle-stimulating hormone (FSH), which helps ovarian follicles develop. Each follicle contains an immature egg. Usually, one follicle becomes dominant and continues towards maturity.

As the follicle develops, it produces increasing amounts of oestrogen. Oestrogen helps the uterine lining grow again after menstruation and plays an important role in preparing the body for ovulation. 1

The length of this phase can vary considerably. This is one reason a period may arrive earlier or later than expected.

3. Ovulation: When an Egg Is Released

Ovulation occurs when an ovary releases an egg. It is triggered by a surge in luteinising hormone (LH), which follows changes in oestrogen levels.

Ovulation is often described as occurring on day 14 of a 28-day cycle. However, this is only an illustration—not a rule. Ovulation does not always occur on day 14, even in people whose cycles are usually regular.

It often occurs approximately 10–16 days before the next period, although timing varies. 1

Some people notice clear, slippery, stretchy cervical mucus around ovulation. Others may notice mild pelvic discomfort or no obvious symptoms at all.

After release, the egg is usually capable of being fertilised for about 12–24 hours. Sperm can survive for several days in the reproductive tract, so the fertile window extends beyond the day of ovulation. 4

Ovulation is an important part of fertility, but noticing a symptom or receiving a positive ovulation test does not guarantee that pregnancy will occur.

4. The Luteal Phase: Preparing for a Possible Pregnancy

After ovulation, the emptied follicle becomes a temporary structure called the corpus luteum. It produces progesterone and some oestrogen.

Progesterone helps maintain the uterine lining in a state that can support implantation if an embryo develops. Some people also experience breast tenderness, bloating, changes in appetite, fatigue or mood symptoms during this phase. 1

If pregnancy does not occur, the corpus luteum regresses and progesterone and oestrogen levels fall. The uterine lining can no longer be maintained in the same way, and menstruation begins. If pregnancy occurs, hormonal signals help maintain the uterine lining rather than allowing a typical menstrual period to begin.

Which Hormones Control the Menstrual Cycle?

The menstrual cycle is regulated through communication between the hypothalamus and pituitary gland in the brain and the ovaries. This system is often called the hypothalamic–pituitary–ovarian axis.

Four hormones are especially important:

1. Follicle-stimulating hormone (FSH): Helps ovarian follicles develop during the first part of the cycle.
2. Luteinising hormone (LH): Its mid-cycle surge triggers ovulation.
3. Oestrogen: Helps the uterine lining grow and contributes to the hormonal signals leading to ovulation.
4. Progesterone: Rises after ovulation and helps prepare and maintain the uterine lining for a possible pregnancy.

These hormones do not work independently. Their levels rise and fall in a coordinated pattern, and the timing of one change influences the next. 1

Other hormones and body systems also matter. Thyroid hormones, prolactin, energy availability, stress-related physiological changes and certain medications can influence menstrual function. This is why menstrual changes cannot always be explained by one hormone or one blood-test result.

How Does the Menstrual Cycle Affect Mood, Energy and Sleep?

You may notice that your energy, appetite, sleep, concentration or mood changes at different points in your cycle. These experiences are real, but they do not follow the same pattern for everyone.

Hormonal fluctuations may contribute to symptoms such as breast tenderness, bloating, headaches, irritability or low mood, particularly in the days before menstruation. Premenstrual syndrome (PMS) refers to a pattern of physical or emotional symptoms that occur before a period and improve around or after its onset. 5

Some people experience little change. Others experience symptoms that interfere with work, relationships, sleep or daily activities.

It is also important not to attribute every change in mood or energy to hormones. Sleep deprivation, anaemia, thyroid conditions, mental health concerns, stress and other health factors can cause similar symptoms.

If symptoms are severe, recur predictably before periods or significantly affect your life, discuss them with a healthcare professional. A symptom diary over two or three cycles may help clarify the pattern.

What Is a Normal Menstrual Cycle?

There is no single cycle length that every healthy person must have. Normality depends on more than the number of days between periods.

For adults in the reproductive years, useful reference points include:

• Cycle length: FIGO uses approximately 24–38 days as the normal frequency range for adults aged 18–45.
• Bleeding duration: Bleeding generally lasts several days. FIGO considers up to eight consecutive days within its normal duration range, while ACOG patient guidance commonly describes a typical period as lasting up to seven days.
• Regularity: Some variation from one cycle to the next is expected. A persistent change from your usual pattern may deserve assessment.
• Flow: Bleeding can be light, moderate or heavier, but bleeding that disrupts daily life or causes symptoms of anaemia should not be ignored. 2 3

These are reference ranges, not a diagnostic checklist. Age, the years since the first period, hormonal contraception, pregnancy, breastfeeding and the transition towards menopause can all affect bleeding patterns.

What About Irregular Periods?

An irregular period does not automatically mean that you have PCOS/PMOS. Changes can occur for several reasons, including adolescence, perimenopause, pregnancy, thyroid disorders, changes in body weight, inadequate energy intake, intense exercise, stress and some medications.

PCOS/PMOS is one possible cause of irregular or absent ovulation, but it cannot be diagnosed from irregular periods alone. Assessment depends on the wider clinical picture and appropriate diagnostic criteria. If your cycles are repeatedly very short, very long, unpredictable or absent, a clinician can help determine whether investigation is needed.

How Can You Track Your Menstrual Cycle?

Tracking can help you understand your own pattern and provide useful information during a medical appointment. You do not need an elaborate app or expensive testing to begin.

Record:

1. The first day of bleeding: This is cycle day 1.
2. How long bleeding lasts: Note light and heavy days.
3. Flow: Record whether bleeding seems light, moderate or heavy and whether you need to change products unusually often.
4. Symptoms: Note cramps, headaches, bowel changes, breast tenderness, mood symptoms and their effect on daily activities.
5. Changes between periods: Record spotting, unusual discharge or pain.
6. Relevant context: Note medication changes, hormonal contraception, major illness or other changes that may affect your cycle.

If you are trying to conceive, cervical mucus observations and ovulation predictor kits may provide additional clues. However, calendar-based apps estimate ovulation; they cannot guarantee the exact day, especially when cycles are irregular. 4

Tracking should help you understand your body, not make you anxious about every variation. If it increases stress, simplify it to the information that is genuinely useful.

Can You Get Pregnant During Your Period?

Pregnancy is less likely during menstruation than around ovulation, but it is not impossible.

Sperm can survive for several days in the reproductive tract. If ovulation occurs early—particularly in a shorter cycle—sperm from sex during or near the end of a period may still be present when an egg is released. Cycle timing also varies, even in people who usually have regular periods. 4

If you are trying to conceive, the fertile window is approximately the six days ending on ovulation, with the highest probability of conception generally in the days just before ovulation. If you are trying to avoid pregnancy, period timing alone is not a reliable contraceptive method.

What Can Change Your Menstrual Cycle?

A change in cycle pattern is a signal to consider the wider context, not a diagnosis by itself.

Common influences include:

1. Adolescence: Periods can be irregular during the first few years after menstruation begins as the reproductive hormone system matures. Persistent or concerning symptoms still need assessment. 6
2. Stress and major life changes: Physiological stress can affect the hormonal signals involved in ovulation, particularly when combined with illness, sleep disruption or inadequate nutrition.
3. Weight and energy availability: Significant weight changes, restrictive eating or exercise that exceeds available energy can disrupt ovulation.
4. PCOS/PMOS: This condition can cause infrequent periods or irregular ovulation.
5. Thyroid disorders and raised prolactin: These conditions can affect menstrual function and may require specific testing and treatment.
6. Hormonal contraception: Some methods change bleeding patterns, reduce bleeding or stop periods altogether. This effect depends on the method used.
7. Pregnancy and breastfeeding: Periods may stop during pregnancy and may return unpredictably after childbirth, particularly during breastfeeding.
8. Perimenopause: Hormonal changes during the transition towards menopause can make periods less predictable.

A clinician may also consider other conditions depending on your symptoms, medical history and examination. Not everyone with an irregular cycle needs the same tests.

When Should You See a Doctor About Your Periods?

Consider arranging a medical assessment if:

• Your periods repeatedly occur outside the expected range for your age and circumstances.
• Your periods stop for three months or longer and you are not pregnant or using a treatment that explains the absence of bleeding.
• Bleeding lasts longer than usual, repeatedly lasts more than seven to eight days, or is heavy enough to interfere with everyday life.
• You bleed between periods or after sex.
• Period pain is severe, worsening or affecting work, sleep, school or daily activities.
• You have persistent pelvic pain, unusual discharge or other symptoms alongside menstrual changes.
• You have very irregular periods and are trying to conceive.
• You feel unusually tired, breathless, dizzy or weak, particularly with heavy bleeding.

Seek urgent medical attention for very heavy bleeding accompanied by faintness, severe dizziness, chest pain or difficulty breathing. If pregnancy is possible and you experience bleeding with severe abdominal pain, shoulder-tip pain or fainting, seek urgent assessment.

The appropriate investigation depends on your age, symptoms and history. It may include a pregnancy test, blood tests, examination or ultrasound; not everyone needs every test. 2 7

The Menstrual Cycle Across Different Life Stages

The menstrual cycle changes throughout life, and the same pattern should not be expected at every age.

1. Adolescence: The first period, called menarche, usually occurs during puberty. Cycles may be irregular at first, but prolonged gaps, very heavy bleeding or significant symptoms warrant attention.
2. Reproductive years: Cycles may become more predictable, although normal variation continues. Pregnancy, contraception, health conditions and lifestyle factors can change the pattern.
3. Pregnancy and postpartum: Menstruation stops during pregnancy. After childbirth, the return of periods varies and can be influenced by breastfeeding. Ovulation may occur before the first postpartum period, so pregnancy can be possible before bleeding returns.
4. Perimenopause: Periods may become less predictable, and changes in sleep, mood, temperature regulation and vaginal or urinary health may occur.
5. Menopause: Menopause is generally recognised after 12 consecutive months without a period when there is no other explanation. Any bleeding after menopause should be assessed by a healthcare professional.

Understanding the stage of life you are in helps put menstrual changes into context, but it should never be used to dismiss symptoms that concern you.

Key Takeaways

• The menstrual cycle involves coordinated hormonal changes in the brain, ovaries and uterus—not just the days of bleeding.
• Day 1 is the first day of your period, and cycle length is counted to the day before your next period.
• The four main stages are menstruation, the follicular phase, ovulation and the luteal phase.
• A 28-day cycle is an example, not a universal standard.
• Ovulation timing varies, and period apps cannot guarantee the exact day.
• Mood, sleep and physical symptoms may change across the cycle, but not everyone experiences the same pattern.
• Tracking your periods and symptoms can help you recognise changes and communicate with your doctor.
• Persistent irregularity, heavy bleeding, severe pain or bleeding after menopause deserves medical assessment.

Your menstrual cycle is not a test you have to pass every month. It is one part of your health, and understanding its patterns can help you make informed decisions without treating every variation as a problem.

You do not need to know every hormone level or predict every period perfectly. Start by understanding your usual pattern, paying attention to symptoms that affect your life, and seeking care when something does not seem right.

References

[1] National Health Service (NHS). Periods and fertility in the menstrual cycle.

[2] American College of Obstetricians and Gynecologists (ACOG). Amenorrhea: Absence of Periods.

[3] Munro MG, Critchley HOD, Fraser IS, et al. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. International Journal of Gynecology & Obstetrics.

[4] American Society for Reproductive Medicine (ASRM). Optimizing Natural Fertility: A Committee Opinion. Fertility and Sterility. 2022;117(1):53–63.

[5] National Health Service (NHS). Premenstrual syndrome (PMS).

[6] American College of Obstetricians and Gynecologists (ACOG). Your First Period.

[7] American College of Obstetricians and Gynecologists (ACOG). Heavy and Abnormal Periods.