Menstrual Cycle: How Your Period Works

What Is the Menstrual Cycle?

The menstrual cycle is a monthly series of changes the body goes through to prepare for pregnancy. Each cycle, the ovaries release an egg (ovulation) while the uterus builds a thick lining. If pregnancy doesn't occur, the lining sheds as menstrual bleeding, and the cycle begins again.

The menstrual cycle is one of the most remarkable examples of biological coordination in the human body. It represents a continuous dialogue between the brain, ovaries, and uterus, orchestrated by a sophisticated hormonal signaling system. This cycle typically begins during puberty, usually between ages 10 and 16, and continues until menopause, which generally occurs between ages 45 and 55.

Understanding how the menstrual cycle works is fundamental to reproductive health awareness. The cycle involves much more than just the period itself - it encompasses a complete series of physiological changes that affect virtually every system in the body, from mood and energy levels to skin health and cognitive function. These changes follow a predictable pattern, though individual variations are entirely normal.

The length of a menstrual cycle is measured from the first day of one period to the first day of the next. While 28 days is often cited as the "average" cycle length, research shows that only about 15% of women actually have exactly 28-day cycles. The normal range spans from 21 to 35 days, with most cycles falling somewhere between 25 and 30 days. Variations of a few days from cycle to cycle are completely normal and do not indicate any underlying health problems.

The purpose of the menstrual cycle, from a biological perspective, is to prepare the body for pregnancy. Each month, the ovaries mature and release an egg, while the uterus builds a nutrient-rich lining capable of supporting a fertilized egg. When pregnancy doesn't occur, this preparation is shed, and the process begins anew. This monthly renewal represents the body's ongoing readiness for reproduction throughout the fertile years.

The main components of the cycle

The menstrual cycle involves three main anatomical structures working in harmony: the hypothalamus and pituitary gland in the brain, the ovaries, and the uterus. The hypothalamus acts as the master controller, releasing hormones that signal the pituitary gland. The pituitary, in turn, releases hormones that stimulate the ovaries. The ovaries then produce their own hormones that prepare the uterus for potential pregnancy while also providing feedback to the brain, completing the communication loop.

What Are the Four Phases of the Menstrual Cycle?

The menstrual cycle consists of four phases: menstruation (days 1-5) when the uterine lining sheds, the follicular phase (days 1-13) when eggs develop, ovulation (around day 14) when a mature egg is released, and the luteal phase (days 15-28) when the body prepares for pregnancy or the next period.

Understanding the four phases of the menstrual cycle provides valuable insight into the changes happening in your body throughout the month. Each phase has distinct characteristics, hormonal profiles, and associated symptoms. While the timing provided here is based on a typical 28-day cycle, remember that your personal cycle may differ.

Phase 1: Menstruation (Days 1-5)

Menstruation marks the beginning of each new cycle. Day 1 is defined as the first day of menstrual bleeding - not spotting, but actual flow. During this phase, the uterine lining (endometrium) that built up during the previous cycle breaks down and is expelled through the vagina along with blood from the exposed blood vessels.

The average period lasts between 3 and 7 days, with most women experiencing 4 to 5 days of bleeding. Blood loss typically ranges from 30 to 40 milliliters total, though anywhere from 20 to 80 milliliters is considered normal. The first two days usually have the heaviest flow, gradually tapering off.

During menstruation, both estrogen and progesterone levels are at their lowest point. This hormonal state triggers the pituitary gland to begin producing follicle-stimulating hormone (FSH), which signals the start of a new cycle. Many women experience symptoms during menstruation including cramping (caused by uterine contractions that help expel the lining), fatigue, mood changes, and sometimes headaches.

Phase 2: Follicular Phase (Days 1-13)

The follicular phase overlaps with menstruation and continues until ovulation. During this phase, the pituitary gland releases follicle-stimulating hormone (FSH), which stimulates the ovaries to develop several follicles - fluid-filled sacs that each contain an immature egg. Usually, one follicle becomes dominant and continues to mature while the others are reabsorbed.

As the dominant follicle grows, it produces increasing amounts of estrogen. This rising estrogen level has several important effects: it stops the menstrual bleeding, stimulates the uterine lining to begin rebuilding and thickening, changes the cervical mucus to become more hospitable to sperm, and eventually triggers the hormonal surge that causes ovulation.

Many women feel their best during the late follicular phase due to rising estrogen levels. Energy increases, mood tends to be more positive, and cognitive function may feel sharper. The cervical mucus becomes clearer and more stretchy as ovulation approaches, a sign that the body is preparing for potential fertilization.

Phase 3: Ovulation (Around Day 14)

Ovulation is the pivotal event of the menstrual cycle - the release of a mature egg from the ovary. It occurs when estrogen levels reach a threshold that triggers a surge of luteinizing hormone (LH) from the pituitary gland. This LH surge causes the mature follicle to rupture, releasing the egg into the fallopian tube.

The timing of ovulation is often stated as "day 14," but this is only accurate for a textbook 28-day cycle. A more reliable way to estimate ovulation is to count backward: ovulation typically occurs about 14 days before the start of the next period. So in a 30-day cycle, ovulation would occur around day 16, while in a 26-day cycle, it would occur around day 12.

The egg remains viable for fertilization for only about 12 to 24 hours after release. However, because sperm can survive in the female reproductive tract for up to 5 days, the fertile window extends from about 5 days before ovulation to 1 day after. Signs of ovulation can include mild pelvic discomfort (mittelschmerz), clear and stretchy cervical mucus resembling egg whites, a slight rise in basal body temperature, and for some women, increased libido.

Phase 4: Luteal Phase (Days 15-28)

After ovulation, the empty follicle transforms into a structure called the corpus luteum, which produces progesterone - the dominant hormone of the luteal phase. Progesterone prepares the uterine lining to receive and nourish a fertilized egg by increasing its thickness and blood supply.

The luteal phase is remarkably consistent in length, typically lasting 12 to 14 days regardless of overall cycle length. If fertilization and implantation occur, the developing embryo produces hormones that maintain the corpus luteum. If pregnancy doesn't occur, the corpus luteum degenerates, progesterone and estrogen levels drop, and this hormonal withdrawal triggers the shedding of the uterine lining - menstruation.

Many women experience premenstrual symptoms (PMS) during the luteal phase, particularly in the days leading up to menstruation. These can include breast tenderness, bloating, mood changes, food cravings, fatigue, and irritability. The symptoms result from the fluctuating hormones and typically resolve once menstruation begins.

What Hormones Control the Menstrual Cycle?

Four main hormones control the menstrual cycle: Follicle-Stimulating Hormone (FSH) stimulates egg development, Luteinizing Hormone (LH) triggers ovulation, estrogen builds the uterine lining and prepares for ovulation, and progesterone maintains the lining after ovulation. These hormones work in a carefully coordinated feedback system.

The menstrual cycle is orchestrated by a sophisticated hormonal communication system known as the hypothalamic-pituitary-ovarian axis. This system involves constant feedback between the brain and ovaries, with each hormone playing a specific role in the monthly reproductive preparation. Understanding these hormones helps explain why the cycle works the way it does and why disruptions can occur.

The hormonal control of the menstrual cycle represents one of the most elegant examples of biological feedback in the human body. Hormones from the brain stimulate the ovaries, and hormones from the ovaries feed back to regulate the brain's output. This creates a self-regulating system that, when functioning properly, produces remarkably consistent monthly cycles.

Gonadotropin-Releasing Hormone (GnRH)

The cycle begins in the hypothalamus, a region at the base of the brain that serves as the master controller. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses throughout the day. The frequency and amplitude of these pulses vary during different phases of the cycle, sending different signals to the pituitary gland.

GnRH travels through a specialized blood vessel network to reach the pituitary gland, where it stimulates the production and release of the two gonadotropins: FSH and LH. The pulsatile nature of GnRH release is essential - continuous exposure would actually decrease, not increase, pituitary hormone production.

Follicle-Stimulating Hormone (FSH)

FSH, produced by the pituitary gland, is primarily active during the follicular phase. As its name suggests, FSH stimulates the growth and development of ovarian follicles. At the beginning of each cycle, when estrogen and progesterone levels are low, FSH levels rise, recruiting a cohort of follicles to begin maturing.

As the follicles develop, they produce estrogen, which feeds back to the pituitary to suppress FSH production. This negative feedback ensures that usually only one follicle (occasionally two) reaches full maturity - the one that's most sensitive to FSH and produces the most estrogen. FSH deficiency or excess can lead to problems with egg development and ovulation.

Luteinizing Hormone (LH)

LH works alongside FSH during follicle development but takes center stage at ovulation. The LH surge - a dramatic increase in LH that occurs about 24 to 36 hours before ovulation - is the trigger that causes the mature follicle to rupture and release its egg. This surge is so predictable that ovulation predictor kits detect it to identify the fertile window.

After ovulation, LH supports the transformation of the empty follicle into the corpus luteum. The corpus luteum requires ongoing LH stimulation to produce progesterone. If pregnancy occurs, the embryo produces human chorionic gonadotropin (hCG), which mimics LH and maintains the corpus luteum until the placenta can take over hormone production.

Estrogen

Estrogen - primarily estradiol - is produced mainly by the developing follicles in the ovaries. Estrogen levels are low at the start of the cycle and rise progressively during the follicular phase, peaking just before ovulation. This hormone has wide-ranging effects throughout the body.

In the reproductive system, estrogen stimulates the growth of the uterine lining, increases blood flow to reproductive organs, and changes cervical mucus to become more receptive to sperm. It also creates the conditions for the LH surge by reaching a threshold level that switches from suppressing to stimulating LH release. Beyond reproduction, estrogen affects bone health, cardiovascular function, skin elasticity, and brain function.

Progesterone

Progesterone is the dominant hormone of the luteal phase, produced primarily by the corpus luteum after ovulation. While estrogen builds the uterine lining, progesterone transforms it into a secretory state capable of supporting an embryo. Progesterone also raises body temperature slightly - the basis for the basal body temperature method of fertility awareness.

Progesterone has a calming effect on the uterus, preventing contractions that could expel an implanting embryo. It also affects the cervical mucus, making it thick and hostile to sperm - after ovulation, when conception is no longer possible, this helps protect against infection. The fall in progesterone at the end of the luteal phase triggers menstruation.

When Does Ovulation Occur and How Can You Tell?

Ovulation typically occurs about 14 days before your next period begins. Signs include clear, stretchy cervical mucus resembling egg whites, mild pelvic pain (mittelschmerz), a slight rise in basal body temperature, and for some women, increased libido. Ovulation predictor kits detect the LH surge that precedes ovulation by 24-36 hours.

Identifying ovulation is important for both those trying to conceive and those trying to avoid pregnancy, as well as for general reproductive health awareness. Unlike menstruation, which has an obvious external sign, ovulation often occurs with subtle symptoms that many women don't notice unless they're specifically paying attention.

The timing of ovulation is one of the most misunderstood aspects of the menstrual cycle. The common belief that ovulation occurs on day 14 is only accurate for women with exactly 28-day cycles. Because the luteal phase (the time from ovulation to the next period) is relatively consistent at 12-14 days, ovulation timing varies with cycle length. A woman with a 35-day cycle ovulates around day 21, while someone with a 24-day cycle ovulates around day 10.

Physical signs of ovulation

The most reliable physical sign of impending ovulation is changes in cervical mucus. In the days leading up to ovulation, rising estrogen levels cause the cervical mucus to become clear, slippery, and stretchy - often described as resembling raw egg white. This "fertile-quality" mucus helps sperm survive and travel through the reproductive tract. After ovulation, progesterone makes the mucus thick, sticky, and opaque.

About 20% of women experience mittelschmerz - a German word meaning "middle pain" - a mild cramping or sharp sensation on one side of the lower abdomen around ovulation. This pain is thought to result from the follicle stretching before rupture or from the small amount of fluid released when the egg is expelled. The side of the pain corresponds to which ovary is releasing an egg that month.

Temperature-based tracking

Basal body temperature (BBT) - your temperature at complete rest, taken immediately upon waking - rises slightly (about 0.2-0.5 degrees Celsius) after ovulation due to progesterone's thermogenic effect. This temperature shift confirms that ovulation has occurred, though it doesn't predict when it will happen. Consistent BBT charting over several months can help identify patterns in your cycle.

Ovulation predictor kits

For a more definitive answer, ovulation predictor kits (OPKs) detect the LH surge in urine. A positive result indicates that ovulation will likely occur within 24-36 hours. These kits are widely available and easy to use, though they work best when you have some idea of when to start testing based on your typical cycle length.

How Can You Track Your Menstrual Cycle?

Track your menstrual cycle by marking the first day of each period (day 1), noting symptoms daily, recording bleeding intensity, and observing ovulation signs like cervical mucus changes. Use a calendar, journal, or smartphone app. Track for at least 3-6 months to identify your personal patterns.

Tracking your menstrual cycle is one of the most valuable things you can do for your reproductive health. It helps you understand your body's patterns, predict your period, identify your fertile window, and recognize changes that might warrant medical attention. Whether you use a simple calendar, a detailed journal, or a sophisticated smartphone app, consistent tracking provides valuable information.

The most basic form of tracking involves recording the first day of each period. Over time, this reveals your average cycle length and how much variation you experience from month to month. More detailed tracking might include daily symptoms, mood changes, cervical mucus observations, basal body temperature, and factors that might influence your cycle like stress, travel, or illness.

What to track

Beyond just marking period start dates, consider tracking: the duration and heaviness of your period, any spotting between periods, physical symptoms like cramps, headaches, or breast tenderness, mood changes throughout the cycle, cervical mucus changes, basal body temperature if you're interested in pinpointing ovulation, and any factors that might affect your cycle. This comprehensive approach provides a fuller picture of your cyclical health.

Modern period-tracking apps can analyze your data to predict upcoming periods and fertile windows, though these predictions are estimates based on averages. Your actual cycle may vary, especially if it's irregular. Apps can be convenient tools, but a simple paper calendar works just as well for basic tracking.

What Is Normal and What Requires Medical Attention?

Normal variations include cycles between 21-35 days, periods lasting 3-7 days, and mild cramping. Seek medical attention for periods longer than 7 days, extremely heavy bleeding, cycles shorter than 21 or longer than 35 days, severe pain interfering with daily life, bleeding between periods, or missing 3+ periods when not pregnant.

One of the most common concerns about menstruation is whether your experience is "normal." The range of normal is actually quite broad, and significant variation exists between individuals. However, certain patterns do warrant medical evaluation, as they may indicate underlying conditions that benefit from treatment.

Normal menstrual cycles range from 21 to 35 days in adults and may be somewhat more variable in the first few years after menarche and in the years approaching menopause. Periods typically last between 3 and 7 days. Some cramping is common, particularly in the first day or two, but should not be severe enough to prevent normal activities. Occasional months with heavier or lighter flow, or cycles that are a few days shorter or longer than usual, are generally not concerning.

When to see a healthcare provider

Consider consulting a healthcare provider if you experience: periods that last longer than 7 days, very heavy bleeding (soaking through a pad or tampon every hour for several consecutive hours, or passing blood clots larger than a quarter), cycles consistently shorter than 21 days or longer than 35 days, severe menstrual pain that doesn't respond to over-the-counter pain relievers or interferes with daily activities, bleeding between periods or after sex, no period for 90 days or more (when not pregnant), or significant changes in your usual pattern.

These symptoms don't necessarily indicate a serious problem, but they deserve evaluation. Many conditions that cause menstrual abnormalities - including polycystic ovary syndrome (PCOS), thyroid disorders, fibroids, and endometriosis - are treatable once identified. Early evaluation can prevent symptoms from worsening and address any underlying health concerns.

What Symptoms Are Common During the Menstrual Cycle?

Common menstrual cycle symptoms include abdominal cramps, breast tenderness, bloating, mood changes, fatigue, headaches, and food cravings. These symptoms vary greatly between individuals and can change throughout life. About 75% of menstruating women experience some premenstrual symptoms, while 3-8% have severe symptoms (PMDD).

Symptoms related to the menstrual cycle extend far beyond the days of menstruation itself. Throughout the month, the fluctuating hormones that drive the cycle can cause physical and emotional changes. Understanding these patterns helps normalize the experience and distinguish between typical symptoms and those that might need medical attention.

The symptoms experienced during the menstrual cycle reflect the body's response to changing hormone levels. Estrogen and progesterone affect not only the reproductive organs but also the brain, breasts, skin, digestive system, and more. This explains why cycle-related symptoms can be so varied and wide-ranging.

Physical symptoms

Menstrual cramps (dysmenorrhea) are among the most common period symptoms, affecting about 80% of menstruating women at some point. Primary dysmenorrhea, the most common type, results from prostaglandins - hormone-like substances that cause the uterus to contract to help expel its lining. These cramps typically begin just before or at the start of menstruation and last 1-3 days.

Breast tenderness often occurs in the luteal phase due to progesterone's effects on breast tissue. Bloating and water retention, also common in the luteal phase, result from hormonal effects on fluid balance. Headaches, including menstrual migraines, can occur in response to the estrogen drop at the end of the cycle. Fatigue is common during menstruation and may relate to hormonal changes, blood loss, or sleep disruptions from cramps.

Emotional and psychological symptoms

Mood changes during the menstrual cycle are extremely common and largely attributed to hormonal fluctuations. Many women report feeling more energetic and positive during the follicular phase when estrogen rises, and more irritable or emotional during the luteal phase when progesterone dominates. These patterns, when mild, are normal variations in mood.

Premenstrual syndrome (PMS) describes a cluster of physical and emotional symptoms occurring in the luteal phase and resolving with menstruation. About 75% of menstruating women experience some PMS symptoms, though severity varies greatly. Common PMS symptoms include irritability, anxiety, depression, difficulty concentrating, food cravings (especially for carbohydrates and chocolate), and sleep disturbances.

A small percentage of women (about 3-8%) experience premenstrual dysphoric disorder (PMDD), a severe form of PMS characterized by significant mood symptoms that interfere with daily functioning. PMDD is recognized as a clinical diagnosis and may benefit from treatment with antidepressants, hormonal contraceptives, or other interventions.

When Does Menstruation Begin and End?

Menstruation typically begins (menarche) between ages 10 and 16, with an average age of 12. It ends with menopause, which occurs on average around age 51 (range 45-55). Perimenopause - the transition to menopause - begins several years earlier and is characterized by irregular cycles and fluctuating hormones.

The menstrual cycle doesn't remain constant throughout life. It has a clear beginning at puberty, often takes time to become regular, may be interrupted by pregnancy and breastfeeding, and eventually ends with menopause. Understanding these transitions helps contextualize the menstrual experience at different life stages.

Menarche: The first period

Menarche, the first menstrual period, typically occurs between ages 10 and 16, with an average age of about 12 in most developed countries. It usually happens 2-3 years after the first signs of puberty appear (breast development and pubic hair growth). The timing of menarche is influenced by genetics, nutrition, body weight, and overall health.

In the first few years after menarche, irregular cycles are very common and usually not a cause for concern. It takes time for the hormonal feedback system to mature and establish regular ovulatory cycles. By the third year after menarche, most adolescents have cycles between 21 and 45 days. Cycles typically become more regular in the late teens and twenties.

Perimenopause and menopause

Perimenopause is the transitional period leading up to menopause, typically beginning in the mid-40s but sometimes earlier. During perimenopause, ovarian function gradually declines, leading to irregular cycles, changing flow, and fluctuating hormones that can cause symptoms like hot flashes, sleep disturbances, and mood changes.

Menopause is officially defined as 12 consecutive months without a menstrual period, marking the end of reproductive capability. The average age of menopause is 51, though it can occur anywhere from the early 40s to the late 50s. Premature menopause (before age 40) affects about 1% of women and may have specific health implications requiring medical attention.