Menstrual cycle is a set of events that occur in a female body every month as it prepares for a possible pregnancy.

This cycle is a series of events in the brain, ovaries, and uterus that are controlled by hormones — chemical signals that travel through the bloodstream from one part of the body to another. These hormonal changes cause the lining of the uterus to grow and thicken, and trigger ovulation, when an ovary releases an egg.

People often use the terms “menstruation” and “menstrual cycle” interchangeably, but they mean different things. Menstruation is the monthly shedding of the lining of the uterus: blood and tissue from the uterus pass through the cervix and out of the body through the vagina. The menstrual cycle includes menstruation, but also all the processes that occur before and after it.

► The menstrual cycle begins on the first day of bleeding and ends one day before the next menstruation begins.

Why Is It Important to Understand the Menstrual Cycle?

The menstrual cycle is part of your daily life, and it affects everything from energy levels and sex drive to mood and overall health.

Understanding the different phases of the menstrual cycle will give you a deeper understanding of your body’s fertility and condition and help you make informed decisions about your daily activities.

When you know when to expect certain changes, you can more confidently plan your exercise routine, special events, or even doctor visits.

Phases of the Menstrual Cycle

The menstrual cycle consists of two interconnected, parallel cycles:

Ovarian cycle (changes in the ovaries)
Uterine cycle (changes in the uterus)

Although these cycles are often described separately, in real time they occur partly simultaneously and act together.

The uterine cycle and the ovarian cycle are divided into different phases. Different events occur in each phase. The ovarian cycle has three phases: follicular (before ovulation), ovulation (when the ovary releases an egg), and luteal (after ovulation). The uterine cycle has four phases: menstruation, proliferative phase, ovulation, and secretory phase (before menstrual bleeding begins).

Ovarian Cycle

This cycle describes the physical changes that occur in the ovaries.

► Note: The following is an example of a 28-day cycle, with ovulation occurring on day 14. Keep in mind that cycle lengths can vary from person to person, and your cycle may not necessarily follow this exact example.

Follicular phase (approximately days #1-14)

When: From the start of menstruation to ovulation.
What happens: Several follicles (fluid-filled sacs that contain eggs) develop in both ovaries. One of these follicles becomes the dominant follicle (the “leading” follicle that most often releases a mature egg during ovulation) and grows to about the size of a button. For most people, this phase lasts about 10 to 22 days, although the length can vary.
Hormones: The pituitary gland (a gland located at the base of the brain) produces follicle-stimulating hormone ( FSH ), which signals the ovaries to prepare an egg for ovulation. The growing dominant follicle produces more and more estrogen (a hormone that helps the lining of the uterus grow and thicken), with levels peaking shortly before ovulation.

Ovulation (approximately day 14)

► Note: Ovulation is part of both the ovarian and uterine cycles, as it is a key event in both.

When: In the middle of the menstrual cycle or about 13-15 days before the start of the next period.
What happens: The dominant follicle releases an egg into the fallopian tube.
Hormones: A sudden “surge” (a sharp increase) of luteinizing hormone (LH) triggers the release of an egg. Soon after ovulation, estrogen levels drop.

Luteal phase (approximately #14-28 days)

When: From ovulation to the start of your next period. This phase often lasts about 14 days, but can range from 9 to 16 days.
What happens: After ovulation, the empty follicle transforms into a corpus luteum, which secretes progesterone (a hormone that prepares the lining of the uterus for the possible implantation of a fertilized egg) and some estrogen to support a potential pregnancy. If pregnancy does not occur, the corpus luteum disintegrates in about 9-11 days.
Hormones: If pregnancy does not occur, progesterone (and to a lesser extent estrogen) levels rise and then fall. These hormonal changes can contribute to typical premenstrual symptoms (which we will discuss in more detail in the next section). When progesterone and estrogen levels fall, menstruation begins again.

How Might I Feel During the Luteal Phase?

During this phase, many people experience premenstrual syndrome (PMS), which can include both emotional and physical changes. Some people have no symptoms at all, while others experience symptoms of varying intensity. Possible signs include:

Mood changes (such as irritability, sadness, or anxiety)
Bloating
Breast tenderness
Mild cramps/pain
Appetite enhancement
Fatigue or changes in energy levels
Sleep disturbance
Headache
Acne

The most common symptom of approaching menstruation is cramping/pain in the pelvic area, which is caused by the uterus contracting as it tries to shed its lining. If the symptoms are so severe that they affect your daily life, see your doctor.

Uterine Cycle

This cycle describes the physical changes taking place in the uterus.

Menstruation (approximately #1-5 days)

When: From the start of menstruation to its end.
What happens: Blood and tissue from the lining of the uterus (endometrium) from the previous cycle are shed through the cervix (the lower part of the uterus that opens into the vagina) and vagina. The endometrium is at its thinnest during this phase. A typical period can last up to 8 days, but it averages about 5-6 days.
Hormones: Estrogen and progesterone levels are usually at their lowest. This causes the top layers of the lining to break down and be shed from the body.

Proliferative phase (approximately #6-14 days)

When: From the end of menstruation to ovulation.
What’s happening: “Proliferative” means growing rapidly. During this phase, the uterus rebuilds and thickens its lining in preparation for a possible pregnancy. At the same time, the ovaries prepare their egg cells (oocytes) for release.
Hormones: During this phase, estrogen levels typically increase. This signals the lining of the uterus to begin growing.

Ovulation (approximately day 14)

► Note: Ovulation is part of both the ovarian and uterine cycles, as it is a key event in both.

When: In the middle of the menstrual cycle, or about 13-15 days before the start of the next period.
What happens: The dominant follicle releases an egg into the fallopian tube.
Hormones: A sudden “surge” (a sharp increase) of luteinizing hormone (LH) triggers the release of an egg. Soon after ovulation, estrogen levels drop.

Secretory phase (approximately #14–28 days)

When: From ovulation to the start of the next menstruation.
What happens: The endometrium thickens further and becomes more nutrient-rich to nourish a fertilized egg in the event of pregnancy. If the fertilized egg does not implant (when the fertilized egg “attaches” to the uterine wall/lining), the uterine lining is shed and menstruation begins.
Hormones: During this phase, progesterone levels rise, causing the lining of the uterus to stop thickening further and prepare for implantation. At the end of the secretory phase, the body releases prostaglandins—hormone-like compounds that can cause cramping/pain—to help shed the lining if pregnancy does not occur.

► After the cycle ends, menstruation will begin again and the cycle will repeat itself.

Menstrual Cycles Vary

One complete menstrual cycle usually lasts between 21 and 35 days (some sources say 24-38 days), although this can vary from person to person and from one cycle to the next.

Most people start menstruating around the age of 12, on average. However, it is possible for menstruating to start as early as 8 or as late as 16. Menstruation usually begins a few years after breast development and pubic hair appear.

Menstruation stops completely during menopause, which occurs on average around the age of 51. During menopause, the body stops producing eggs (i.e., ovulation stops). Menopause is the stage when a person has not had a period for one year.

The length of the cycle may even change over time, for example in the years following menarche (the first menstruation) or as you approach menopause (when menstruation permanently stops).

Sometimes ovulation does not occur at all, which is called anovulation. This is more common in the first few years of menstruation and also later, during perimenopause (the transition to menopause). Anovulation can also occur during breastfeeding or due to certain health problems, such as polycystic ovary syndrome (PCOS) . In some cases, menstruation may seem regular even though ovulation has not occurred, and the cause may remain unknown.

Because hormone levels fluctuate throughout the cycle, some people may notice changes in their hair, skin, digestion (including stool), mood, migraine headaches, or even how they experience sex at different stages of their cycle.

Irregular Menstruation – What Should You Know?

Irregular menstruation refers to any occurrence that does not correspond to a “normal” menstrual period. Some examples of irregular menstruation include:

Menstruation that occurs at intervals of less than 21 days or more than 35 days;
Absence of menstruation for three months (90 days);
Menstrual flow that is much heavier or much lighter than usual;
Bleeding that lasts more than 7 days.
Menstruation accompanied by severe pain, cramps, nausea, or vomiting.
Bleeding or spotting between periods.

During menstruation, you can usually expect to lose about 35 ml of blood, although a range of 10-80 ml is considered normal. Signs of irregular bleeding may include:

Completely wetting a diaper or tampon every 1-2 hours;
The release of large clots of blood.
Bleeding that lasts more than 7 days each time.

► It is normal for the amount of bleeding to vary somewhat from cycle to cycle. It is also important to remember that your “normal” may be different from others. Try not to compare yourself to others. If you are experiencing irregular or very heavy bleeding, see your doctor.

How to Track Your Menstrual Cycle

It’s a good idea to know what your menstrual cycle is like. Not having a period may not seem like a big deal, but irregular periods can sometimes be a sign of a more serious health problem. Your doctor will often ask you about your last period and your menstrual cycle in general, so it’s helpful to know what’s normal for you.

Tracking your menstrual cycle can also help you understand when you’re ovulating — or when you’re most likely to get pregnant. It’s also useful for planning ahead, such as preparing for your period during a special event or vacation.

To keep an eye on your menstrual cycle:

Mark the first day of your period on the calendar with an X. This is the first day of your cycle.
Then mark with an X each day that you bleed. Stop marking when the bleeding stops.
When bleeding starts again, mark it with an X again. This is the first day of a new cycle.
You can then count the days between the first Xs to get the length of your cycle. The number of Xs will tell you how many days your bleeding will last.

► There are mobile applications that will do all this for you.

When Should You See a Doctor?

Some variation in your menstrual cycle is normal, but it’s worth talking to your doctor if you:

You consistently have cycles that are longer than normal (21-35 days);
You have missed your period for more than three months in a row (and you are not pregnant);
You have very heavy or prolonged bleeding (for example, you completely wet a pad or tampon every hour for several hours, or the bleeding lasts for more than 8 days);
You have severe pain or other symptoms that affect your daily life.

Sources: Cleveland Clinic, HelloClue