Menstrual Cycle

The menstrual cycle is the monthly hormonal sequence that prepares the body of a person with a uterus and ovaries for a possible pregnancy. It runs from the first day of one period to the day before the next, lasts roughly 21 to 35 days, and is driven by a coordinated rise and fall of estrogen, progesterone, and two pituitary hormones. Across each cycle the ovaries mature and release an egg while the uterine lining thickens to receive a fertilized one; if no pregnancy occurs, the lining sheds as a period and the sequence begins again. Understanding the phases, the hormones behind them, and what is normal for your own body helps with tracking fertility, anticipating symptoms and mood shifts, and recognizing when something is worth a clinician's attention. This article is general education, not medical advice.

What is the Menstrual Cycle?

The menstrual cycle is a recurring physiological process, typically lasting about 28 days on average, in which hormones prepare the uterus and ovaries for a potential pregnancy. It is counted from the first day of bleeding (the period) to the day before the next period starts. Although "menstruation" and "menstrual cycle" are often used interchangeably in everyday speech, they are not the same thing: menstruation is the bleeding phase, while the menstrual cycle is the entire hormonal loop of which bleeding is only one part.

The cycle is the result of a feedback conversation between the brain and the ovaries. The hypothalamus and pituitary gland in the brain release signaling hormones that tell the ovaries to mature an egg; the ovaries respond by producing estrogen and progesterone, which in turn build and maintain the uterine lining (endometrium) and feed information back to the brain. When the lining is not needed for an implanted embryo, hormone levels drop and the lining is shed. This entire system is sometimes called the hypothalamic-pituitary-ovarian axis.

The menstrual cycle usually begins at puberty, with the first period (menarche) typically occurring between ages 10 and 16, and continues until menopause, usually between ages 45 and 55, when cycles permanently stop. In between, cycles can be interrupted or altered by pregnancy, breastfeeding, hormonal contraception, stress, illness, significant weight change, and a range of medical conditions. No two bodies cycle identically, and a single body's cycles can vary from month to month.

The Four Phases of the Menstrual Cycle

The cycle is conventionally divided into four phases. Some sources describe two main phases (follicular and luteal) split by ovulation, treating menstruation as the start of the follicular phase; this article uses the more detailed four-phase model that most cycle-tracking resources rely on. The day ranges below are approximate and assume a roughly 28-day cycle — your own phase lengths may differ, and the luteal phase tends to be the most consistent in length while the follicular phase varies most.

Menstrual Phase

Days 1–5 (approximately). The menstrual phase is the period itself — the shedding of the uterine lining through the vagina when no pregnancy has occurred. It begins on the first day of bleeding, which is counted as day one of the whole cycle. Bleeding typically lasts three to seven days and consists of blood, endometrial tissue, and mucus, totaling a relatively small volume despite often appearing heavier.

Estrogen and progesterone are at their lowest during this phase. Common physical symptoms include cramps (dysmenorrhea) from the uterus contracting to expel its lining, lower back ache, headache, breast tenderness, bloating, and fatigue. Mood during the period varies widely: some people feel low, irritable, or tearful in the first day or two, while others report relief and renewed energy as premenstrual symptoms lift once bleeding starts.

Follicular Phase

Days 1–13 (approximately). The follicular phase technically overlaps with the period, beginning on day one and ending at ovulation. It is named for the ovarian follicles — small fluid-filled sacs, each containing an immature egg — that begin to develop under the influence of follicle-stimulating hormone (FSH). Usually one follicle becomes dominant and continues maturing while the others break down.

As the dominant follicle grows, it produces increasing amounts of estrogen, which rebuilds and thickens the uterine lining that was shed during the period. This rising estrogen is associated with improving mood, increasing energy, sharper focus, and rising libido toward the end of the phase. Many people feel at their physical and emotional best in the late follicular phase as estrogen climbs toward its peak.

Ovulation

Around day 14 (approximately). Ovulation is the release of a mature egg from the dominant ovarian follicle. It is triggered by a sharp surge in luteinizing hormone (LH), which itself is set off by estrogen reaching its peak. The released egg travels into the fallopian tube, where it can be fertilized by sperm for roughly 12 to 24 hours before it begins to break down.

Ovulation is the single most fertile event of the cycle, but the fertile window is wider than the day of ovulation alone because sperm can survive in the reproductive tract for up to five days. Some people notice ovulation through mild one-sided lower-abdominal twinges (mittelschmerz), a change to clear, stretchy, egg-white-like cervical mucus, a slight rise in libido, or breast tenderness. Mood is often stable or elevated around ovulation thanks to high estrogen.

Luteal Phase

Days 15–28 (approximately). After ovulation, the emptied follicle transforms into a structure called the corpus luteum, which secretes progesterone (and some estrogen). Progesterone maintains and stabilizes the thickened uterine lining, preparing it to support a fertilized egg. The luteal phase is usually the most fixed in length, typically lasting 12 to 14 days.

If the egg is not fertilized, the corpus luteum breaks down, progesterone and estrogen fall sharply, and this hormone withdrawal triggers the next period. The drop is the hormonal basis of premenstrual syndrome (PMS): in the days before bleeding, many people experience bloating, breast tenderness, food cravings, acne, fatigue, irritability, anxiety, or low mood. A more severe form, premenstrual dysphoric disorder (PMDD), causes significant mood disturbance and warrants medical attention. If fertilization does occur, the embryo signals the body to keep producing progesterone, the lining is maintained, and no period follows.

Hormones of the Menstrual Cycle

Four hormones do most of the work, rising and falling in a fixed sequence. This is the pattern often shown on a "menstrual cycle hormone chart": FSH and estrogen lead the first half, an LH surge triggers ovulation in the middle, and progesterone dominates the second half before everything falls to restart the cycle.

Estrogen

Estrogen (chiefly estradiol) is produced by the developing follicle. It rebuilds the uterine lining after a period, drives the LH surge that triggers ovulation, and rises to its highest point just before ovulation. Estrogen also affects mood, energy, skin, and libido, which is why the late follicular phase often feels like a high point. After ovulation it dips, rises again modestly in the luteal phase, then falls before menstruation.

Progesterone

Progesterone is produced by the corpus luteum after ovulation and is the defining hormone of the luteal phase. It stabilizes the uterine lining for possible implantation, raises basal body temperature slightly, and has a calming-to-sedating effect for some people while contributing to PMS symptoms for others. When progesterone falls at the end of an unfertilized cycle, that withdrawal triggers the period.

FSH (Follicle-Stimulating Hormone)

FSH is released by the pituitary gland and, as its name says, stimulates ovarian follicles to grow at the start of the cycle. It is highest early in the follicular phase. As the dominant follicle produces more estrogen, that estrogen feeds back to suppress FSH, helping ensure usually only one egg matures per cycle.

LH (Luteinizing Hormone)

LH is also released by the pituitary. Its defining moment is the mid-cycle LH surge: a sharp spike triggered by peak estrogen that causes the mature follicle to rupture and release its egg, producing ovulation about 24 to 36 hours after the surge begins. Ovulation predictor kits work by detecting this LH rise in urine.

Cycle Length and What Counts as Normal

Counting Your Cycle

Cycle length is counted from the first day of one period (day one, the first day of full bleeding, not spotting) to the day before the next period begins. A common mistake is to count from the end of one period to the start of the next; the correct count always starts on the first day of bleeding. The number of bleeding days and the total cycle length are two separate measurements.

Normal Variation

A typical menstrual cycle lasts between 21 and 35 days in adults, with an average of about 28 days, though "28 days" is a textbook average rather than a target your body must hit. Cycles outside the 21–35 day range, or that vary widely from month to month, are worth tracking and discussing with a clinician. Period bleeding usually lasts three to seven days.

Cycle length naturally varies more in the first few years after menarche and again in the years approaching menopause (perimenopause), when cycles often become irregular. Stress, travel, illness, intense exercise, significant weight change, thyroid problems, and conditions such as polycystic ovary syndrome (PCOS) can all lengthen, shorten, or disrupt cycles. An occasional irregular cycle is common and usually not a cause for concern; a persistent pattern of irregularity is worth investigating.

Tracking Your Cycle

Tracking helps you predict your period, identify your fertile window, notice patterns in symptoms and mood, and provide useful information to a clinician. No single method is perfect, and combining methods improves accuracy.

Calendar and Apps

The simplest method is marking the first day of each period on a calendar over several months to learn your average cycle length and predict future periods. Menstrual cycle calculators and tracking apps automate this, estimating your next period and a probable fertile window from your logged dates. These predictions are estimates based on averages and are least reliable for people with irregular cycles. Be aware that many apps store sensitive personal data, so it is worth checking an app's privacy practices.

Basal Body Temperature

Basal body temperature (BBT) is your resting temperature taken first thing in the morning before getting up. Progesterone raises BBT by roughly 0.3–0.5°C (about 0.5–1°F) after ovulation, so a sustained temperature rise confirms that ovulation has already happened. BBT charting is useful for understanding your luteal phase and confirming ovulation, but because it shows ovulation only after the fact, it does not predict the fertile window in advance and works best combined with other signs.

Cervical Mucus and Other Signs

Cervical mucus changes predictably across the cycle: scant and dry after menstruation, then increasingly wet, clear, and stretchy (like raw egg white) around ovulation, which signals peak fertility. After ovulation it becomes thicker and cloudier. Ovulation predictor kits detect the LH surge in urine and can flag the one to two most fertile days in advance. Tracking mucus, BBT, and cycle dates together — sometimes called the symptothermal method — gives the fullest picture.

The Cycle and Fertility

Conception depends on sperm meeting a released egg in the fallopian tube. Because the egg survives only about 12 to 24 hours after ovulation but sperm can live up to five days in the reproductive tract, the fertile window spans roughly the five days before ovulation plus the day of ovulation itself — about six days per cycle. The highest chance of conception is in the two to three days leading up to and including ovulation.

For people trying to conceive, identifying this window through cycle tracking, ovulation predictor kits, and cervical mucus observation can help time intercourse. For people trying to avoid pregnancy without hormonal methods, fertility-awareness-based methods use the same signs to identify and avoid the fertile window — but these methods require careful, consistent tracking and have higher failure rates than many other forms of contraception when not used meticulously. Fertility also declines with age, particularly after the mid-30s, independent of cycle regularity.

Common Irregularities and When to See a Clinician

Most people experience an irregular cycle occasionally, and that alone is rarely concerning. Patterns that are worth discussing with a clinician include: cycles consistently shorter than 21 days or longer than 35 days; periods that stop for three or more months without pregnancy (amenorrhea); very heavy bleeding that soaks through protection hourly or includes large clots (menorrhagia); bleeding between periods or after sex; periods that become noticeably more painful or change pattern; and severe pain that interferes with daily life, which can be a sign of conditions such as endometriosis.

Other reasons to seek care include severe premenstrual mood symptoms that disrupt your life (possible PMDD), signs of conditions like PCOS or thyroid disorders, and any sudden change from your established personal pattern. A clinician may use a menstrual history, blood tests, and imaging such as ultrasound to investigate. None of these signs is a self-diagnosis; they are reasons to get evaluated. This article is educational and does not replace individualized medical advice.

The Cycle and Contraception

Most hormonal contraception works by interrupting the natural cycle. Combined pills, patches, and vaginal rings supply steady estrogen and progestin that suppress the FSH and LH signals, preventing ovulation; the regular "bleed" during the placebo week of many combined pills is a withdrawal bleed, not a true period, and is not medically necessary. Progestin-only methods (the mini-pill, hormonal IUDs, the implant, and the injection) work mainly by thickening cervical mucus and thinning the uterine lining, and may stop ovulation; they often make periods lighter, less frequent, or absent.

Because these methods override the natural hormonal sequence, cycle tracking based on natural signs is not meaningful while using most hormonal contraception. After stopping, it can take a few weeks to several months for natural cycles to return, depending on the method (the injection in particular can delay return of fertility). Non-hormonal options, such as the copper IUD and barrier methods, do not alter the menstrual cycle, though the copper IUD can make periods heavier. Choosing a method involves weighing effectiveness, side effects, and personal preferences with a clinician.

Examples

A person trying to conceive tracks their cycle for three months, learns their average length is 30 days, and uses ovulation predictor kits to detect the LH surge around day 16. They time intercourse to the two days before and the day of the positive test, which is their most fertile window.

Someone notices their mood reliably dips and they feel bloated and irritable in the four or five days before their period, then lifts once bleeding starts. Recognizing this as a luteal-phase, PMS-type pattern, they plan demanding commitments for the higher-energy follicular phase and discuss the pattern with their clinician.

A person with cycles that swing between 24 and 45 days and occasional skipped months tracks the irregularity in an app and brings the record to a clinician, who investigates for PCOS. The tracking data makes the consultation more productive.

Someone using a combined contraceptive pill notices their "periods" are lighter and perfectly regular. Their clinician explains these are withdrawal bleeds caused by the placebo week rather than true menstrual periods, and that the pill is suppressing their natural cycle and ovulation.

See Also

• Ovulation
• Menstruation
• Follicular Phase
• Luteal Phase
• Pregnancy
• Hormonal Contraception
• Vagina

FAQ

What are the four phases of the menstrual cycle?

The four phases are the menstrual phase (the period, roughly days 1–5, when the uterine lining sheds), the follicular phase (roughly days 1–13, when ovarian follicles mature and estrogen rebuilds the lining), ovulation (around day 14, when a mature egg is released), and the luteal phase (roughly days 15–28, when progesterone maintains the lining and PMS symptoms may appear before the next period). The day ranges assume a 28-day cycle and vary between bodies; the luteal phase tends to be the most consistent in length while the follicular phase varies most.

How long is a normal menstrual cycle?

A typical adult menstrual cycle lasts between 21 and 35 days, with an average of about 28 days. Cycle length is counted from the first day of one period to the day before the next begins. Cycles are often more irregular in the years after the first period and approaching menopause. Cycles consistently shorter than 21 days or longer than 35 days, or that vary widely month to month, are worth discussing with a clinician.

Which hormones control the menstrual cycle?

Four main hormones drive the cycle. FSH (follicle-stimulating hormone) stimulates follicles to grow early in the cycle; estrogen, made by the maturing follicle, rebuilds the uterine lining and peaks before ovulation; LH (luteinizing hormone) surges mid-cycle to trigger ovulation; and progesterone, made by the corpus luteum after ovulation, maintains the lining during the luteal phase. When progesterone and estrogen fall at the end of an unfertilized cycle, that withdrawal triggers the period.

When am I most fertile in my cycle?

You are most fertile in the days leading up to and including ovulation, which occurs around the middle of the cycle (about day 14 in a 28-day cycle). Because sperm can survive up to five days and the egg lives about 12–24 hours after release, the fertile window spans roughly six days: the five days before ovulation plus ovulation day. The highest chance of conception is in the two to three days just before and including ovulation. Ovulation predictor kits and egg-white-like cervical mucus help identify this window.

How do mood and symptoms change across the cycle?

Many people feel lowest energy and some discomfort (cramps, fatigue) during the period, then increasingly energetic, focused, and upbeat through the follicular phase as estrogen rises, often feeling their best around ovulation. In the luteal phase, after ovulation, falling hormones can bring premenstrual symptoms — bloating, breast tenderness, cravings, irritability, anxiety, or low mood — that typically ease once the period starts. Patterns vary widely between individuals; severe premenstrual mood symptoms that disrupt daily life (possible PMDD) deserve medical attention.

How do I calculate my menstrual cycle?

Count cycle length from the first day of full bleeding (day one) to the day before your next period starts — not from the end of one period to the start of the next. Track this over several months to find your average length, then estimate your next period by adding that average to your last period's start date. Menstrual cycle calculators and apps automate this. To estimate your fertile window, ovulation typically occurs about 14 days before the next expected period, with the fertile window in the five days before it. These are estimates and are less reliable with irregular cycles.

When should I see a doctor about my cycle?

Consider seeing a clinician if your cycles are consistently shorter than 21 days or longer than 35 days, if periods stop for three or more months without pregnancy, if bleeding is very heavy or includes large clots, if you bleed between periods or after sex, if periods become much more painful or change pattern, or if premenstrual mood symptoms severely disrupt your life. Any sudden change from your established personal pattern is also worth evaluating. These signs are reasons to get assessed, not self-diagnoses, and this article is general education rather than individualized medical advice.