11 min read · Updated September 2026 · Educational content only — not medical advice
Your menstrual cycle is more than the days you bleed. Across the cycle, coordinated changes in hormones, the ovaries and the uterine lining prepare the body for a possible pregnancy — and start the process again if pregnancy does not occur.
The menstrual cycle begins on the first day of menstrual bleeding and ends when the next period begins. During that time, hormones coordinate changes in the ovaries and uterus: follicles develop, an egg may be released, the uterine lining prepares for a possible pregnancy and — if pregnancy does not occur — hormone levels fall and the lining is shed.
You will often see this described as four phases:
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Menstruation
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The follicular phase
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Ovulation
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The luteal phase
That model is useful, but it is a simplification.
In reproductive physiology, the ovarian cycle is more formally divided into the follicular and luteal phases, with ovulation between them. Menstruation actually occurs during the beginning of the follicular phase. The uterus has its own overlapping pattern of menstrual, proliferative and secretory changes.
You do not need to memorize both systems. The four-phase model works well for understanding the cycle as long as you remember that the phases overlap biologically rather than switching like four separate calendar blocks.
In short
Day 1 is the first day of your period, not the day after bleeding ends.
The commonly used four phases are menstruation, follicular, ovulation and luteal.
Estrogen rises mainly before ovulation, while progesterone becomes dominant after ovulation.
Ovulation does not automatically happen on day 14. Timing varies between people and between cycles.
A 28-day cycle is an example, not a requirement for a healthy menstrual cycle.
The hormones behind the menstrual cycle
Four hormones are particularly useful for understanding what is happening:
Follicle-stimulating hormone (FSH) is released by the pituitary gland and helps ovarian follicles develop.
Estrogen, particularly estradiol, rises as a dominant follicle develops. Among other effects, it helps rebuild the uterine lining after menstruation.
Luteinizing hormone (LH) is also released by the pituitary gland. A sharp rise in LH — the LH surge — helps trigger ovulation.
Progesterone rises after ovulation and helps transform the uterine lining into an environment capable of supporting implantation if fertilization occurs.
These hormones work through feedback loops between the brain, pituitary gland and ovaries rather than acting independently.
You do not need perfectly predictable hormone levels to have a normal cycle, and hormone concentrations do not follow exactly the same curve in every person.
Phase 1: Menstruation
Menstruation is the part of the cycle you can see most clearly: the uterine lining is shed and leaves the body through the vagina as menstrual fluid.
But the process actually begins before bleeding appears.
During the previous luteal phase, progesterone and estrogen help maintain the uterine lining. If pregnancy does not occur, the structure in the ovary producing much of that progesterone — the corpus luteum — breaks down. Estrogen and progesterone levels fall, and that hormonal withdrawal triggers the lining to shed.
The first day of full menstrual bleeding is cycle day 1.
ACOG states that a typical menstrual period lasts around 2 to 7 days.
During menstruation, estrogen and progesterone are relatively low. At the same time, however, the ovaries are already beginning the next part of the cycle.
That is why menstruation is not really a completely separate ovarian phase: the follicular phase has already begun.
What you may notice
Bleeding is the defining feature, but some people also experience:
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lower-abdominal cramping
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lower-back pain
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headaches
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fatigue
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bloating
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digestive changes
The intensity varies widely. Significant pain is not required for menstruation to be “normal.”
Phase 2: The follicular phase
The follicular phase begins on day 1 of the cycle and continues until ovulation.
Its name comes from the ovarian follicles — small fluid-filled structures that contain immature eggs.
Early in the cycle, FSH helps stimulate a group of follicles to develop. As they grow, they produce increasing amounts of estradiol. Eventually, one usually becomes the dominant follicle, while the others stop developing.
At the same time, rising estrogen affects the uterus.
After the menstrual lining has been shed, estrogen stimulates the endometrium — the lining of the uterus — to grow again. This is known as the proliferative phase of the uterine cycle.
Cervical mucus also changes as ovulation approaches. It generally becomes wetter, clearer and more receptive to sperm around the fertile period. ACOG notes that observing cervical mucus can be one way of identifying approaching ovulation.
Why this phase changes the length of your cycle
The follicular phase is typically the more variable part of the menstrual cycle.
If the dominant follicle takes longer to reach the point where ovulation can occur, ovulation happens later — and the entire cycle becomes longer.
Research following naturally cycling women has repeatedly found substantial variability in follicular-phase length both between individuals and from one cycle to another.
This is one major reason calendar predictions based on a fixed “day 14” can be misleading.
Good to know
Your period ending does not mark the start of the follicular phase.
Biologically, the follicular phase already started on day 1 of bleeding. The common four-phase model separates menstruation mainly because it is useful for understanding and tracking the cycle.
Phase 3: Ovulation
Ovulation is the release of an egg from the dominant follicle in an ovary.
As that follicle matures, estradiol rises substantially. Sustained high estrogen eventually changes its feedback effect on the brain and pituitary gland, producing a sharp rise in LH.
This LH surge helps trigger the final steps leading to follicle rupture and release of the egg.
Does ovulation happen on day 14?
Sometimes — but day 14 is not a biological rule.
The familiar day-14 example comes from dividing a textbook 28-day cycle roughly in half. In reality, cycle lengths and follicular phases vary.
ACOG explains that ovulation typically occurs approximately 12 to 14 days before the next period begins, rather than necessarily 14 days after the previous period started.
In a shorter cycle, ovulation may therefore occur earlier. In a longer cycle, it may occur considerably later.
Research in regularly cycling women has also shown that the fertile period can occur across a much wider range of cycle days than the traditional “days 10–17” calendar assumption suggests.
What happens around ovulation?
Around this time:
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an egg is released from the ovary
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cervical mucus tends to become more favorable to sperm
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some people notice a small rise in basal body temperature after ovulation
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some experience temporary one-sided pelvic discomfort known as mittelschmerz
NICHD notes that ovulation timing and these physical signs vary between people.
The egg itself remains capable of fertilization for a relatively short period. Sperm can survive longer in the reproductive tract, which is why the fertile window begins several days before ovulation. ACOG describes potential conception from approximately five days before ovulation through about one day afterward.
That estimate is useful for understanding fertility, but simple calendar counting should not be treated as a guarantee of either pregnancy or contraception.
Phase 4: The luteal phase
After ovulation, the empty follicle does not immediately disappear.
Its remaining cells reorganize into a temporary structure called the corpus luteum. The corpus luteum produces progesterone, along with some estrogen.
Progesterone now becomes the dominant hormone of this phase.
Its job includes transforming the estrogen-built uterine lining into a more secretory, nutrient-rich environment that could support implantation of a fertilized egg.
Progesterone also slightly raises basal body temperature, which is why fertility-awareness methods can sometimes use a sustained temperature rise as retrospective evidence that ovulation has occurred.
If pregnancy does not occur
Without pregnancy, the corpus luteum eventually regresses.
Progesterone and estrogen levels fall. The uterine lining loses its hormonal support and begins to break down, leading to the next menstrual period.
The cycle starts again at day 1.
If pregnancy does occur
After fertilization and early implantation, the developing pregnancy begins producing human chorionic gonadotropin, or hCG.
hCG signals the corpus luteum to continue producing progesterone during early pregnancy rather than breaking down on its usual schedule.
That prevents the hormone withdrawal that would otherwise trigger menstruation.
Where PMS fits into the cycle
Premenstrual syndrome is associated with the luteal phase, particularly the days leading up to menstruation.
PMS can involve physical and emotional symptoms such as bloating, breast tenderness, headaches, irritability, mood changes and difficulty concentrating. ACOG defines PMS as a recurring pattern of symptoms that occurs before menstruation.
That does not mean everyone experiences a predictable emotional or physical state in every phase.
Online descriptions sometimes assign specific personality traits, productivity levels, diets or exercise styles to each phase of the cycle. Hormonal changes are real, but individual symptoms vary considerably, and the evidence does not support treating every person as if they should feel the same way on the same cycle day.
Tracking your own pattern is more useful than trying to fit your experience into a predetermined template.
How long should a menstrual cycle be?
Twenty-eight days is useful for diagrams. It is not the definition of a normal cycle.
ACOG commonly describes menstrual cycles in adults as lasting approximately 21 to 35 days, while the U.S. Office on Women's Health uses a slightly narrower typical range of 24 to 38 days. Different clinical references therefore use somewhat different boundaries.
More important than matching 28 days exactly is understanding your usual pattern and noticing substantial or persistent changes.
Cycle length can also vary from one month to the next. Research has shown meaningful within-person variation even among people considered to have regular menstrual cycles.
Why cycles vary
Cycle timing can be influenced by factors including:
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age and stage of reproductive life
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stress
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illness
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substantial changes in weight or energy intake
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intense exercise
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pregnancy and breastfeeding
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perimenopause
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hormonal contraception
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some medical conditions
An occasional longer or shorter cycle does not automatically indicate a medical problem.
A persistent change is more informative.
What if you use hormonal birth control?
The four-phase explanation above describes a naturally ovulatory menstrual cycle.
Some hormonal contraceptives deliberately change that process.
Combined hormonal methods such as many birth control pills, the patch and the vaginal ring prevent pregnancy primarily by suppressing ovulation. They also change cervical mucus and thin the uterine lining.
That means someone using combined hormonal contraception does not necessarily move through the same follicular → ovulation → luteal sequence described above.
Bleeding during a hormone-free or placebo interval can also be a withdrawal bleed rather than menstruation resulting from a natural ovulatory cycle. ACOG notes that this scheduled withdrawal bleeding is not biologically necessary for health.
Other methods affect ovulation differently, so the exact pattern depends on the contraception being used.
When to talk to a healthcare professional
Variation is part of menstrual biology, but certain patterns are worth discussing with an ob-gyn or another healthcare professional.
Consider seeking advice if:
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your cycles are repeatedly much shorter or longer than your usual pattern
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your periods become significantly more irregular
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bleeding lasts longer than about 7 days
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you bleed or spot between periods
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your bleeding becomes unusually heavy
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you stop having periods for several months without an expected reason such as pregnancy
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period pain or other symptoms significantly interfere with everyday life
ACOG considers patterns such as cycles consistently shorter than 21 days or longer than 35 days, substantial cycle-to-cycle variation, bleeding between periods and prolonged or very heavy bleeding reasons for evaluation.
Those signs can have many causes. They do not diagnose a particular condition on their own.
The bottom line
The menstrual cycle is a continuous hormonal process, not simply the days you have your period.
The familiar four-phase model — menstruation, follicular phase, ovulation and luteal phase — is a useful way to understand what changes throughout the month. Just remember that the biology overlaps: menstruation occurs during the beginning of the follicular phase, and ovulation is an event separating the follicular and luteal portions of the ovarian cycle.
Your cycle also does not need to follow a perfect 28-day schedule. Ovulation can shift, the follicular phase is naturally variable and cycle length can change from month to month.
Understanding the pattern is useful. Expecting the body to follow an exact calendar usually is not.
Sources & References
American College of Obstetricians and Gynecologists (ACOG). The Menstrual Cycle: Menstruation, Ovulation, and How Pregnancy Occurs. Updated 2025.
ACOG — The Menstrual Cycle
American College of Obstetricians and Gynecologists (ACOG). Amenorrhea: Absence of Periods.
ACOG — Amenorrhea: Absence of Periods
American College of Obstetricians and Gynecologists (ACOG). Abnormal Uterine Bleeding.
ACOG — Abnormal Uterine Bleeding
Office on Women's Health, U.S. Department of Health and Human Services. Your Menstrual Cycle.
Office on Women's Health — Your Menstrual Cycle
Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). About Menstruation.
NICHD — About Menstruation
Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, Menstrual Cycle. StatPearls. Updated 2024. National Library of Medicine.
NCBI Bookshelf — Physiology, Menstrual Cycle
Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. Endotext. National Library of Medicine.
NCBI Bookshelf — The Normal Menstrual Cycle and the Control of Ovulation
Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. Journal of Obstetric, Gynecologic & Neonatal Nursing. 2006;35(3):376–384.
PubMed — Variability in the phases of the menstrual cycle
