Understanding Your Menstrual Cycle and Ovulation

What Is the Menstrual Cycle?

The menstrual cycle is the monthly process your body goes through to prepare for pregnancy. It involves a complex interplay between your brain, ovaries, and uterus, all coordinated by hormones. The cycle begins on the first day of your period (menstrual bleeding) and ends the day before your next period starts.

While we often refer to a "28-day cycle" as the standard, this is just an average. According to the American College of Obstetricians and Gynecologists (ACOG), normal menstrual cycles can range from 21 to 35 days in adults and 21 to 45 days in teens who have recently started menstruating. Your personal cycle length may also vary by a few days from month to month—this is completely normal. For a detailed breakdown of exactly when ovulation occurs relative to your period, see our article on how many days after your period you ovulate.

The Purpose of the Menstrual Cycle

The fundamental purpose of the menstrual cycle is to prepare your body for pregnancy each month. During each cycle, your body:

• Develops and releases a mature egg (ovulation)
• Prepares the uterine lining (endometrium) to receive a fertilized egg
• If pregnancy doesn't occur, sheds the uterine lining (menstruation)
• Starts the process again with a new cycle

The Two Ovaries

You have two ovaries, one on each side of your uterus. During each cycle, typically only one ovary releases an egg, though which ovary does so can vary from cycle to cycle. It's not necessarily alternating—the same ovary might release the egg several months in a row. Occasionally, both ovaries release an egg in the same cycle, which is how fraternal twins can occur.

At birth, your ovaries contain all the eggs you'll ever have—approximately 1-2 million. By puberty, this number decreases to about 300,000-400,000. Of these, only about 400-500 will actually be released through ovulation during your reproductive years, as noted by the NIH's National Institute of Child Health and Human Development (NICHD).

The Hormones That Control Your Cycle

Your menstrual cycle is orchestrated by four main hormones, each with specific roles at different times in your cycle. Understanding these hormones helps explain why you may experience certain symptoms at different times of the month.

Follicle-Stimulating Hormone (FSH)

FSH is produced by the pituitary gland in your brain. As its name suggests, it stimulates the follicles in your ovaries. At the beginning of each cycle, FSH levels rise, signaling several follicles to begin developing. Each follicle contains an immature egg. Usually, only one follicle will become dominant and release its egg; the others will be reabsorbed by the body.

Luteinizing Hormone (LH)

LH is also produced by the pituitary gland. While it's present in small amounts throughout your cycle, it surges dramatically about 24-36 hours before ovulation. This LH surge triggers the dominant follicle to release its egg. Ovulation predictor kits (OPKs) work by detecting this LH surge in your urine. To learn more about recognizing when this surge happens, read our guide to the signs of ovulation.

Estrogen

Estrogen is primarily produced by the developing follicles in your ovaries during the first half of your cycle. As follicles grow, estrogen levels rise, which:

• Thickens the uterine lining to prepare for potential implantation
• Changes cervical mucus to become more sperm-friendly
• Eventually triggers the LH surge that causes ovulation
• May increase energy, mood, and libido

Progesterone

Progesterone is produced by the corpus luteum—the structure that remains after the follicle releases its egg. Progesterone levels rise after ovulation and:

• Maintain the thickened uterine lining
• Raise your basal body temperature slightly
• Change cervical mucus to become thicker and less penetrable
• May cause PMS symptoms in some women

If pregnancy occurs, progesterone continues to be produced to support the pregnancy. If not, progesterone levels drop, triggering menstruation. For a detailed look at this timing, see how long after ovulation your period starts.

The Hypothalamic-Pituitary-Ovarian (HPO) Axis

Behind every menstrual cycle is a sophisticated communication network between three key structures in your body: the hypothalamus (a small region at the base of the brain), the anterior pituitary gland (just below the hypothalamus), and the ovaries. Together, they form the hypothalamic-pituitary-ovarian (HPO) axis—the master control system that regulates your reproductive hormones and, ultimately, ovulation and menstruation.

Understanding the HPO axis helps explain not only how a normal cycle works, but also why things like stress, extreme exercise, eating disorders, and conditions like polycystic ovary syndrome (PCOS) can disrupt your periods and fertility.

How the HPO Axis Works

The cycle begins in the hypothalamus, which secretes gonadotropin-releasing hormone (GnRH) in a pulsatile pattern. These pulses travel a short distance to the pituitary gland, where they stimulate the release of FSH and LH into the bloodstream. FSH and LH then travel to the ovaries and drive follicle development, estrogen production, and eventually ovulation.

What makes this system so elegant is its feedback loops. In the early follicular phase, low levels of estrogen exert negative feedback on the hypothalamus and pituitary, keeping FSH and LH at moderate levels. But as the dominant follicle matures and estrogen levels climb past a critical threshold, something remarkable happens: estrogen switches to positive feedback. This triggers the massive LH surge that causes ovulation—a process that, according to a review in Physiological Reviews (Herbison, 2015), involves complex interactions between estrogen, kisspeptin neurons, and GnRH pulses.

After ovulation, progesterone produced by the corpus luteum exerts strong negative feedback on the HPO axis, preventing another LH surge and additional ovulation during the same cycle. This is also the principle behind hormonal birth control methods, which use synthetic hormones to keep the HPO axis in a suppressed state.

Positive vs. Negative Feedback

The switch between negative and positive feedback is one of the most important concepts in reproductive endocrinology. Here is a simplified summary:

• Negative feedback (most of the cycle): Estrogen and progesterone suppress GnRH, FSH, and LH release, preventing premature ovulation and keeping the cycle orderly.
• Positive feedback (late follicular phase only): When estrogen from the dominant follicle exceeds approximately 200 pg/mL for about 50 hours, it triggers a surge of GnRH and LH rather than suppressing them. This is the signal for ovulation.

Disruptions at any level of the HPO axis can affect your cycle. For example, chronic psychological stress elevates cortisol, which can suppress GnRH pulses from the hypothalamus and delay or prevent ovulation entirely. Similarly, conditions that affect the pituitary gland (such as elevated prolactin from a benign pituitary tumor) or the ovaries (such as PCOS or premature ovarian insufficiency) can alter this finely tuned feedback system. If you have irregular periods, understanding the HPO axis can help you and your healthcare provider identify where the disruption might be occurring.

Phase 1: The Menstrual Phase (Days 1-5)

The menstrual phase is what most people think of as their "period." It's the beginning of a new cycle, marked by the shedding of the uterine lining that built up during the previous cycle.

What Happens During Menstruation

When pregnancy doesn't occur in the previous cycle, the corpus luteum breaks down and stops producing progesterone. This drop in progesterone levels signals the body that the uterine lining is no longer needed. The blood vessels that supply the endometrium constrict, causing the tissue to break down and shed through the vagina.

Menstrual flow consists of blood, tissue from the uterine lining, and mucus. The amount of blood lost during a typical period is about 2-3 tablespoons (30-45 mL), though it can range from 1 tablespoon to 5 tablespoons and still be considered normal, according to the Cleveland Clinic.

Duration and Flow

Most periods last 3-7 days, with 5 days being average. Flow is typically heaviest during the first 2 days and then gradually lightens. You may notice that menstrual blood varies in color from bright red (fresh blood) to dark brown or even black (older blood that has oxidized).

Common Symptoms

During the menstrual phase, you may experience:

• Cramping: Caused by uterine contractions that help expel the lining. Mild to moderate cramping is normal.
• Lower back pain: Often accompanies uterine cramping
• Fatigue: Low hormone levels and blood loss can contribute to feeling tired
• Headaches: May be triggered by hormonal changes
• Mood changes: Some women feel relieved as PMS symptoms subside; others may feel low energy

Fertility During Menstruation

While fertility is generally low during menstruation, it's not impossible to conceive from intercourse during your period, especially if you have a short cycle. Sperm can survive up to 5 days in the female reproductive tract, so if you have a 21-day cycle and ovulate around day 7, sperm from intercourse on day 3-4 of your period could potentially still be viable. For more details on the timing of your most fertile days, see our guide to your fertile window.

Phase 2: The Follicular Phase (Days 1-13)

The follicular phase begins on the first day of your period and lasts until ovulation. It's called the "follicular" phase because it's when follicles in your ovaries develop and mature.

Follicle Development

At the beginning of your cycle, your pituitary gland releases FSH, which stimulates multiple follicles to start growing. Each follicle is a fluid-filled sac containing an immature egg. Typically, 10-20 follicles begin developing, but usually only one will become the "dominant follicle" that fully matures and releases its egg.

As follicles develop, they produce increasing amounts of estrogen. The dominant follicle is usually the one that produces the most estrogen. The rising estrogen levels eventually suppress FSH production, which causes the other follicles to stop developing and be reabsorbed.

Uterine Changes

While follicles develop in the ovaries, estrogen stimulates the uterine lining to thicken. The endometrium grows from about 1-2 mm after menstruation to about 8-14 mm before ovulation. This thick, nutrient-rich lining will support a fertilized egg if pregnancy occurs.

Cervical Mucus Changes

As estrogen rises during the follicular phase, your cervical mucus changes progressively:

• Early follicular phase: Little to no mucus, vagina feels dry
• Mid-follicular phase: Sticky, tacky mucus, white or cream colored
• Late follicular phase: Creamy, lotion-like mucus
• Approaching ovulation: Clear, stretchy, egg-white-like mucus (peak fertility)

Variable Length

The follicular phase is the variable part of your cycle. While the luteal phase (after ovulation) is relatively consistent at about 14 days, the follicular phase can vary significantly. This is why women with longer cycles ovulate later, and why factors like stress can delay your period by delaying ovulation.

How You Might Feel

Many women report feeling their best during the late follicular phase. As estrogen rises:

• Energy levels typically increase
• Mood often improves
• Mental clarity may be enhanced
• Libido often increases as ovulation approaches
• Skin may look clearer and more radiant

Phase 3: Ovulation (Day 14)

Ovulation is the main event of your menstrual cycle—the release of a mature egg from the ovary. This is the only time during your cycle when pregnancy can occur.

The LH Surge and Egg Release

When estrogen levels reach a certain threshold, they trigger a surge in luteinizing hormone (LH) from the pituitary gland. This LH surge causes the dominant follicle to rupture and release its mature egg. The egg is captured by the finger-like projections (fimbriae) at the end of the fallopian tube and begins its journey toward the uterus.

The actual moment of ovulation happens quickly—the egg is released within about 24-36 hours after the LH surge begins. This is why ovulation predictor kits, which detect the LH surge, can give you advance warning that ovulation is imminent.

The Egg's Journey

After release, the egg travels down the fallopian tube toward the uterus. This journey takes about 6-12 hours. Fertilization, if it occurs, typically happens in the upper portion of the fallopian tube within the first 12-24 hours after ovulation. The egg is only viable for about 12-24 hours, so timing is critical for conception.

Physical Signs of Ovulation

Many women experience physical signs of ovulation:

• Cervical mucus peak: Clear, stretchy, egg-white-like mucus that can stretch an inch or more between fingers
• Ovulation pain (mittelschmerz): A twinge or ache on one side of the lower abdomen, lasting minutes to hours
• Increased libido: Many women notice heightened sexual desire
• Slight spotting: Some women experience light spotting when the follicle ruptures
• Breast tenderness: May begin around ovulation
• Heightened senses: Some women report enhanced sense of smell

Timing Ovulation

In a "textbook" 28-day cycle, ovulation occurs on day 14. However, the key to predicting ovulation is understanding that it typically occurs about 14 days before your next period—not 14 days after your last one. For example:

• 28-day cycle: Ovulation around day 14
• 30-day cycle: Ovulation around day 16
• 26-day cycle: Ovulation around day 12
• 35-day cycle: Ovulation around day 21

Use our ovulation calculator to estimate your ovulation date based on your personal cycle length.

Phase 4: The Luteal Phase (Days 15-28)

The luteal phase begins immediately after ovulation and continues until your next period. This phase is named after the corpus luteum, the structure that forms from the empty follicle after the egg is released.

The Corpus Luteum

After ovulation, the empty follicle transforms into the corpus luteum (Latin for "yellow body" due to its color). This temporary gland produces progesterone and some estrogen. Progesterone is essential for:

• Maintaining the thickened uterine lining
• Creating a receptive environment for embryo implantation
• Preventing further ovulation during this cycle
• Supporting early pregnancy if conception occurs

If Pregnancy Occurs

If the egg is fertilized and implants in the uterine lining, the developing embryo begins producing human chorionic gonadotropin (hCG). This hormone signals the corpus luteum to keep producing progesterone, which is essential for maintaining the pregnancy. The corpus luteum continues this role until the placenta takes over hormone production around 10-12 weeks of pregnancy.

If Pregnancy Doesn't Occur

If fertilization doesn't happen, or if a fertilized egg doesn't successfully implant, the corpus luteum begins to break down about 10-14 days after ovulation. Progesterone and estrogen levels drop, which triggers:

• The blood vessels supplying the endometrium to constrict
• The uterine lining to break down
• Menstruation to begin, starting a new cycle

Luteal Phase Length

Unlike the follicular phase, the luteal phase is relatively consistent, typically lasting 12-16 days with 14 days being average. A luteal phase shorter than 10 days may indicate a luteal phase defect, which can make it difficult for a fertilized egg to implant successfully. If you suspect a short luteal phase, consult your healthcare provider. Our article on how long after ovulation your period arrives explains this timing in greater detail.

Premenstrual Symptoms (PMS)

Many women experience premenstrual symptoms during the luteal phase, typically in the week before their period. These can include:

• Mood changes (irritability, anxiety, depression)
• Bloating and water retention
• Breast tenderness
• Food cravings
• Fatigue
• Headaches
• Acne breakouts

These symptoms are thought to be related to the hormonal fluctuations, particularly the rise and fall of progesterone. Symptoms typically resolve once menstruation begins.

Cycle Variations and What's Normal

While we've described a "typical" 28-day cycle, there's actually a wide range of normal when it comes to menstrual cycles.

Normal Cycle Length

A normal menstrual cycle can range from 21 to 35 days in adults. For teenagers, cycles can be 21 to 45 days as their bodies establish regular patterns. It's also normal for your cycle length to vary by a few days from month to month. A large-scale 2019 study published in NPJ Digital Medicine analyzed over 600,000 menstrual cycles and confirmed that only about 13% of cycles are exactly 28 days.

When Cycles Are Considered Irregular

Your cycles may be considered irregular if:

• They're consistently shorter than 21 days or longer than 35 days
• The length varies by more than 7-9 days from cycle to cycle
• You sometimes skip periods entirely (when not pregnant)
• Your period lasts less than 2 days or more than 7 days

If your cycles are frequently unpredictable, our guide on irregular periods and ovulation offers practical strategies for tracking fertility when your cycle doesn't follow the textbook.

Factors That Affect Cycle Regularity

Many factors can temporarily or chronically affect your menstrual cycle:

• Age: Cycles are often irregular for the first few years after menstruation begins and again as women approach menopause
• Weight changes: Significant weight loss or gain can disrupt ovulation
• Stress: Physical or emotional stress can delay ovulation
• Exercise: Excessive exercise, especially combined with low body weight, can affect cycles
• Medical conditions: PCOS, thyroid disorders, and other conditions can cause irregular cycles
• Hormonal birth control: It may take several months for cycles to regulate after stopping birth control

When to See a Doctor

Consult a healthcare provider if you experience:

• No period by age 15
• Sudden changes in your cycle pattern
• Periods that stop for more than 90 days (when not pregnant)
• Extremely heavy bleeding or periods lasting longer than 7 days
• Severe pain that interferes with daily activities
• Bleeding between periods or after sex

Our article on when to see a fertility specialist offers additional guidance on age-based timelines and warning signs.

How Birth Control Affects Your Cycle

Hormonal birth control is one of the most common influences on the menstrual cycle, and understanding how it works can help you plan for the future—whether you're starting a new method, considering switching, or preparing to stop contraception and conceive. Different methods affect ovulation and menstruation in distinct ways, and the time it takes for your natural cycle to resume can vary.

Combined Oral Contraceptives (The Pill)

Combined pills contain synthetic estrogen and progestin. They work primarily by suppressing the HPO axis—preventing the LH and FSH surges needed for follicle development and ovulation. The "period" you have during the placebo week is actually withdrawal bleeding, not a true menstrual period triggered by ovulation. According to the NHS, most women resume ovulation within 1-3 months of stopping the pill, though it can occasionally take longer.

Progestin-Only Methods

Progestin-only pills (the "mini pill"), hormonal IUDs (such as Mirena or Kyleena), and the implant (Nexplanon) primarily work by thickening cervical mucus and thinning the uterine lining. Some of these methods also suppress ovulation partially or fully, depending on the dose. With the hormonal IUD, many women continue to ovulate but have lighter or absent periods because the progestin keeps the uterine lining thin.

Injectable Contraceptives

Depo-Provera (the birth control shot) is given every three months and suppresses ovulation completely in most users. It is notable for having the longest return-to-fertility time among reversible methods. According to Planned Parenthood, it can take 6-12 months (sometimes up to 18 months) for regular ovulation and cycles to return after stopping the shot.

Non-Hormonal Methods

The copper IUD (Paragard) does not contain hormones and does not suppress ovulation. Instead, it creates an inflammatory environment in the uterus that is toxic to sperm. Because it does not affect the HPO axis, your cycle continues normally while using it, and fertility returns immediately upon removal.

If you are planning to transition from birth control to trying to conceive, it can be helpful to start tracking your cycle with our ovulation calculator once your periods resume, so you can identify your fertile window as quickly as possible.

Your Menstrual Cycle as a Vital Sign

In 2015, the American College of Obstetricians and Gynecologists (ACOG) published a landmark committee opinion recommending that clinicians treat the menstrual cycle as a vital sign—just like blood pressure, heart rate, and temperature. This was a significant statement: it acknowledged that the regularity and characteristics of your period can reveal important information about your overall health, not just your reproductive health.

What Your Cycle Can Tell You

Changes in your menstrual cycle can be early indicators of a range of conditions. For example:

• Irregular or absent periods can signal hormonal imbalances, thyroid dysfunction, PCOS, or hypothalamic amenorrhea caused by excessive stress or exercise.
• Unusually heavy periods (soaking through a pad or tampon in an hour for several consecutive hours) may indicate uterine fibroids, polyps, or bleeding disorders such as von Willebrand disease.
• Cycles shorter than 21 days or longer than 35 days may suggest anovulation, luteal phase defects, or the early stages of perimenopause.
• Severe menstrual pain that does not respond to over-the-counter medication may warrant evaluation for endometriosis or adenomyosis.

According to the Mayo Clinic, tracking your cycle consistently gives both you and your healthcare provider a valuable baseline. When something changes—whether it is cycle length, flow volume, pain levels, or premenstrual symptoms—you will notice it sooner and can seek evaluation before small issues become bigger problems.

When Irregular Cycles Warrant Attention

While occasional cycle variation is normal, certain patterns deserve a conversation with your doctor:

• Consistently irregular cycles after age 20 (when post-puberty variation should have settled)
• Amenorrhea (no period for 3 or more months) in someone who previously menstruated regularly
• Significant changes in cycle pattern that coincide with weight changes, new medications, or stressors
• Any postmenopausal bleeding

By tracking your cycle and being aware of what is normal for you, you are taking a proactive step in safeguarding your health. Explore our full library of fertility and cycle articles for more ways to stay informed.

How to Track Your Cycle

Tracking your menstrual cycle provides valuable insights into your fertility, helps you predict your period, and can alert you to potential health issues. Here are the main methods:

Calendar Tracking

The simplest method is to mark the first day of your period on a calendar each month. Over time, you'll see patterns in your cycle length. You can use our ovulation calculator to estimate your ovulation based on your cycle data.

Basal Body Temperature (BBT)

Your BBT is your temperature at complete rest. Taking your temperature every morning before getting up can help confirm ovulation. After ovulation, progesterone causes BBT to rise by about 0.5-1°F and stay elevated until your next period. Use a BBT thermometer for accuracy and take your temperature at the same time each day. For a complete guide, read our article on basal body temperature and ovulation tracking.

Cervical Mucus Monitoring

Checking your cervical mucus daily can help identify your fertile window. As ovulation approaches, mucus becomes clear, stretchy, and slippery. After ovulation, it becomes thicker and less noticeable.

Ovulation Predictor Kits (OPKs)

These urine tests detect the LH surge that occurs 24-36 hours before ovulation. They're helpful for timing intercourse when trying to conceive.

Fertility Apps

Many smartphone apps can help you track your cycle, symptoms, and fertility signs in one place. Look for apps that allow you to log multiple types of data and provide predictions based on your personal patterns.

Combining Methods

The most accurate approach is to combine multiple tracking methods. Using calendar tracking with cervical mucus observation and either BBT or OPKs can give you a comprehensive picture of your cycle and fertility. Research published in the Journal of Obstetric, Gynecologic & Neonatal Nursing supports combining methods for more reliable ovulation confirmation.

Hormone Levels Throughout Your Cycle

Understanding the approximate hormone levels during each phase of your cycle can help you make sense of the physical and emotional changes you experience. The table below summarizes the four primary reproductive hormones, their typical ranges in each phase, and the role each hormone plays. Keep in mind that normal ranges can vary between laboratories and between individuals, so these values represent general reference ranges as reported by the NIH's National Institute of Child Health and Human Development (NICHD).

Tracking how these hormones interact explains many cycle-related experiences. For instance, the rise in estrogen during the follicular phase is why many women feel energized and optimistic before ovulation, while the progesterone dominance of the luteal phase can contribute to PMS symptoms. To learn how to monitor one of these hormonal effects at home, see our guide to basal body temperature and ovulation tracking.

The Cellular Process of Ovulation

Ovulation is far more than a single moment when an egg pops out of the ovary. At the cellular level, it is a carefully orchestrated sequence of events that begins weeks before the egg is released and involves hundreds of signaling molecules. Here is a step-by-step visual timeline of how ovulation unfolds at the cellular level.

Cycle Irregularity Patterns

Not every irregular cycle is cause for concern, but recognizing specific patterns can help you and your healthcare provider identify underlying issues. The table below outlines four common cycle irregularity patterns, what might be causing them, and when to take action.

Age-Related Changes to Your Cycle

Your menstrual cycle is not static--it evolves throughout your reproductive life. Understanding what to expect at different ages can help you distinguish normal changes from signs that warrant medical attention. The American College of Obstetricians and Gynecologists (ACOG) and the Cleveland Clinic both provide excellent resources on these life-stage changes.

Teens (Ages 12-19): Establishing a Rhythm

Most girls get their first period (menarche) between ages 10 and 15, with the average around age 12. For the first 2-3 years, cycles are often irregular because the HPO axis is still maturing. Cycles may range from 21 to 45 days, and anovulatory cycles are common during this period. According to ACOG, by age 3 years post-menarche, most teens should have cycles within the 21-35 day range. If regular periods have not established by age 16, or if a teen has not started menstruating by age 15, evaluation is recommended.

20s: Peak Regularity

For most women, the 20s represent the most regular and predictable menstrual cycles. The HPO axis is fully mature, ovulation occurs consistently, and cycles tend to fall within a narrow range each month. Fertility is at its peak during this decade. Cycle lengths typically average 26-30 days with predictable ovulation. This is often considered the biological benchmark for "normal" cycles.

30s: Subtle Shifts Begin

During the 30s, particularly the late 30s, some women begin to notice gradual changes. Ovarian reserve (the number of remaining eggs) declines, and the quality of remaining eggs also decreases. FSH levels may begin to rise slightly as the ovaries require more stimulation to develop follicles. Cycles may begin to shorten by a day or two as the follicular phase becomes shorter. These changes are subtle and may go unnoticed by many women, but they are the early signs of reproductive aging. The Cleveland Clinic notes that perimenopause can begin as early as the mid-30s in some women, though this is less common.

40s: Perimenopause Begins

Most women enter perimenopause in their 40s, the transition phase that leads to menopause. During perimenopause, hormonal fluctuations become more pronounced. You may experience shorter cycles, then longer cycles, then skipped periods. Ovulation becomes less predictable--some months you may ovulate, others you may not. Flow may change too, with periods becoming heavier or lighter than your established pattern. Other common symptoms include hot flashes, night sweats, sleep disruption, and mood changes. Perimenopause can last 4-8 years on average. Despite irregular ovulation, pregnancy is still possible during perimenopause, so contraception remains important if pregnancy is not desired.

Late 40s-50s: Approaching Menopause

Menopause is defined as going 12 consecutive months without a menstrual period. The average age of menopause in the United States is 51, according to the ACOG, but it can occur anytime between 45 and 55. In the final years before menopause, periods may become very irregular, with gaps of several months between them. Hormone levels fluctuate widely, and anovulatory cycles become increasingly common. Once menopause is reached, ovulation has ceased and natural pregnancy is no longer possible.

Hormone Fluctuations Throughout the Cycle

The four key reproductive hormones -- FSH, LH, estrogen, and progesterone -- rise and fall in a carefully orchestrated pattern throughout each menstrual cycle. According to the World Health Organization (WHO), disruptions to these hormonal patterns are among the leading causes of infertility worldwide. Visualising how these hormones interact helps explain why specific symptoms appear at specific times in your cycle and how tracking methods like OPKs and BBT charting work.

Relative Hormone Levels by Cycle Phase (28-Day Cycle)

During the early follicular phase (days 1-5), all hormones are at their lowest levels. FSH then rises to recruit and develop follicles. As the dominant follicle grows, it produces increasing amounts of estrogen, which peaks just before ovulation around day 12-13. This estrogen peak triggers the LH surge -- the signal detected by ovulation predictor kits -- which causes ovulation approximately 24-36 hours later. After ovulation, progesterone becomes the dominant hormone as the corpus luteum forms, reaching its peak around 7 days post-ovulation (day 21 in a 28-day cycle). If pregnancy does not occur, the corpus luteum degenerates, all hormones drop, and menstruation begins. The Cleveland Clinic provides an excellent clinical overview of this hormonal cascade.

Understanding these patterns has practical implications. The estrogen rise before ovulation causes fertile cervical mucus production and is why you notice wetter, stretchier mucus as ovulation approaches. The progesterone rise after ovulation explains the BBT temperature shift, breast tenderness, and mood changes of the luteal phase. For a detailed look at what happens at each stage, see our menstrual cycle phases explained guide.

How Cycle Length Varies and Why

While 28 days is often cited as the "standard" cycle length, research published by the National Institute for Health and Care Excellence (NICE) and large-scale tracking studies show that cycle length varies considerably among women and even between cycles in the same woman. According to data from over 600,000 cycles analysed by Symul et al. (2019), the median cycle length is approximately 29 days, with 95% of cycles falling between 21 and 40 days.

Common Causes of Cycle Length Variation

Cycle Length Distribution

The critical insight is that virtually all cycle length variation occurs in the follicular phase (before ovulation), not the luteal phase (after ovulation). This is why knowing when you ovulate is far more useful than knowing your cycle length for predicting your period or timing conception. If your cycles are consistently outside the 21-35 day range, or if your cycle length varies by more than 7-9 days from month to month, consider discussing this with your healthcare provider. Our guide on irregular periods and ovulation provides detailed strategies for tracking fertility when cycles are unpredictable.

The Menstrual Cycle and Overall Health

Your menstrual cycle is increasingly recognised by medical organisations as a vital sign -- as important as blood pressure, pulse, and temperature for assessing overall health. The March of Dimes and ACOG both recommend that healthcare providers routinely ask about menstrual patterns, particularly for adolescents and women of reproductive age. Changes in your cycle can be early warning signs for a range of health conditions beyond reproductive health.

What Your Cycle Can Tell You About Your Health

Your cycle health is intimately connected to your cardiovascular, metabolic, and bone health. Women with chronic anovulation (no ovulation) lose the protective effects of regular progesterone production, which can increase the risk of endometrial hyperplasia over time. Similarly, the loss of regular estrogen cycling -- as seen in hypothalamic amenorrhea -- can contribute to bone density loss even in young women. For women actively trying to conceive, cycle health indicators also provide important clues about fertility potential and whether medical evaluation may be helpful.

If you are concerned about what your cycle may be telling you about your health, start by tracking at least 3-6 months of data using a fertility tracking app. Record cycle lengths, flow heaviness, pain levels, and any symptoms like acne or mood changes. This data is extremely valuable when you discuss your cycle with a healthcare provider. For women with signs of ovulation that are unclear or absent, or those with cycles that fall outside normal ranges, our guide on when to see a fertility specialist provides specific guidance on when to seek help.

Related Reading

Explore our complete library of fertility and cycle health guides:

• Ovulation Calculator -- Free tool to estimate your fertile window
• About Ovulation Calculator -- Learn about our mission and methodology
• Menstrual Cycle Phases Explained
• Signs of Ovulation: How to Know When You're Ovulating
• Your Fertile Window: Best Days to Conceive
• How Many Days After Your Period Do You Ovulate?
• How Long After Ovulation Does Your Period Start?
• Ovulation With Irregular Periods
• Cervical Mucus and Fertility
• Basal Body Temperature and Ovulation Tracking
• OPK Ovulation Predictor Kits Guide
• Clearblue Ovulation Calculator Guide
• Best Fertility Tracker Apps
• Irregular Periods and Ovulation
• When to See a Fertility Specialist
• Due Date Calculation Methods
• Due Date Calculator from Ovulation

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