The menstrual cycle is a repeating series of hormonal and physical changes that prepares the body for a possible pregnancy. It involves the ovaries, uterus, brain, and several hormones working together. The cycle begins on the first day of menstrual bleeding and continues until the day before the next period.
Although a 28-day cycle is often used as a standard example, menstrual cycles vary considerably from person to person and can also change over time. The important events are the same: an egg-containing follicle develops in an ovary, ovulation may release an egg, the uterine lining thickens in preparation for pregnancy, and that lining is shed if pregnancy does not occur.
The four phases of the menstrual cycle
The menstrual cycle is commonly divided into four phases: menstruation, the follicular phase, ovulation, and the luteal phase. These phases overlap in timing because menstruation occurs at the beginning of the follicular phase.
Menstruation
Menstruation is the bleeding phase of the cycle. It begins on day one of the cycle, when the uterine lining starts to break down and leave the body through the vagina.
This happens because hormone levels, particularly progesterone and estrogen, have fallen after the previous cycle did not result in pregnancy. Without the hormonal support that maintained the uterine lining, the lining is shed.
Menstrual bleeding commonly lasts several days, although the duration varies. Cramps can occur because the uterus contracts to help expel its lining. Prostaglandins, hormone-like substances involved in inflammation and smooth-muscle contraction, contribute to these contractions and can also contribute to menstrual pain.
The follicular phase
The follicular phase begins on the first day of menstruation and continues until ovulation. During this phase, follicles in an ovary begin developing. A follicle is a small fluid-filled structure that contains an immature egg.
The brain helps regulate this process through hormones. The hypothalamus releases gonadotropin-releasing hormone, which signals the pituitary gland to release follicle-stimulating hormone, or FSH. FSH encourages ovarian follicles to grow.
As follicles develop, they produce increasing amounts of estrogen. Usually, one follicle becomes dominant and continues developing toward ovulation, while the others stop developing.
Estrogen also acts on the uterus. As its level rises, the uterine lining, called the endometrium, begins rebuilding and becomes thicker after menstruation.
Ovulation
Ovulation is the release of a mature egg from an ovary. It occurs when a sustained rise in estrogen triggers a sharp increase in luteinizing hormone, or LH, from the pituitary gland. This is known as the LH surge.
The LH surge causes the mature follicle to release its egg. The egg then enters the fallopian tube, where it can potentially encounter sperm and be fertilized.
Ovulation does not necessarily occur on day 14. The commonly cited 28-day cycle is only an example, and the timing of ovulation can differ between individuals and from one cycle to another.
Around ovulation, some people notice changes such as clearer, more slippery cervical mucus. Some also experience mild pelvic pain or cramping, sometimes called mittelschmerz.
The luteal phase
After ovulation, the follicle that released the egg changes into a temporary hormone-producing structure called the corpus luteum.
The corpus luteum produces progesterone and also releases some estrogen. Progesterone helps maintain and further prepare the uterine lining so that it can support a potential pregnancy.
If the released egg is not fertilized and pregnancy does not occur, the corpus luteum eventually breaks down. Progesterone and estrogen levels then fall. The uterine lining loses hormonal support and begins to shed, starting the next menstrual period.
If pregnancy occurs, hormonal signals from the developing pregnancy maintain the corpus luteum during early pregnancy, allowing progesterone production to continue until the placenta becomes the main source of progesterone.
How hormones control the cycle
The menstrual cycle is coordinated through communication between the brain and reproductive organs.
The hypothalamus in the brain releases gonadotropin-releasing hormone. This stimulates the pituitary gland to produce FSH and LH. These hormones act on the ovaries, where they regulate follicle development, estrogen production, and ovulation.
Estrogen and progesterone then act on the uterus and also feed information back to the brain and pituitary gland. This feedback helps regulate the timing and intensity of hormone production during different parts of the cycle.
FSH is particularly important early in the cycle because it supports the development of ovarian follicles. Estrogen rises as a dominant follicle develops. A sufficiently high and sustained estrogen level helps trigger the LH surge, which leads to ovulation. After ovulation, progesterone becomes the dominant hormone of the luteal phase.
These changing hormone levels are responsible for many of the physical changes associated with the menstrual cycle.
What symptoms can occur during the cycle?
Menstrual symptoms vary widely. Some people experience few noticeable changes, while others have significant symptoms during one or more phases of the cycle.
Menstrual cramps
Menstrual cramps, or dysmenorrhea, are caused largely by contractions of the uterus. Prostaglandins help trigger these contractions, which can produce pain in the lower abdomen or pelvis.
Pain may also spread to the lower back or thighs. Cramps are often strongest around the beginning of menstruation and tend to improve as bleeding continues.
Bloating and breast tenderness
Hormonal changes during the cycle can affect fluid balance and breast tissue. Some people experience bloating, breast tenderness, or a feeling of fullness, particularly during the days before menstruation.
These symptoms are not identical in every cycle and can vary considerably between individuals.
Mood changes
Changes in estrogen and progesterone can affect the brain and may contribute to mood changes around the menstrual cycle. Some people experience irritability, sadness, anxiety, or increased emotional sensitivity before their period.
When these symptoms are severe and consistently interfere with daily life, they may be associated with premenstrual syndrome or, in more severe cases, premenstrual dysphoric disorder.
Changes in cervical mucus
Cervical mucus changes in response to reproductive hormones. As ovulation approaches, rising estrogen generally causes cervical mucus to become wetter, clearer, and more slippery.
After ovulation, increased progesterone causes the mucus to become thicker and less abundant. These changes are part of the reproductive system’s normal response to changing hormone levels.
Changes in energy, appetite, and other symptoms
Some people notice changes in appetite, energy, headaches, sleep, bowel habits, or skin during different parts of the cycle. These symptoms can be influenced by hormonal changes, but they are not experienced by everyone and are not reliable indicators of exactly which phase someone is in.
Why menstrual cycles vary
A menstrual cycle does not have to be exactly 28 days to be normal. Cycle length can vary between people and can also vary for the same person from one cycle to another.
The follicular phase is generally more variable than the luteal phase. Factors such as age, reproductive stage, stress, illness, changes in body weight, intense physical activity, and certain medications or hormonal conditions can affect menstrual patterns.
During adolescence, cycles may be irregular while the reproductive hormone system matures. Menstrual cycles also become irregular during the years leading up to menopause, when ovarian hormone production becomes less predictable.
What happens if pregnancy does not occur?
After ovulation, progesterone prepares the uterine lining for a possible pregnancy. If fertilization and implantation do not lead to an ongoing pregnancy, the hormonal support from the corpus luteum declines.
The resulting fall in progesterone and estrogen contributes to the breakdown of the uterine lining. The tissue and blood leave the body through the vagina as menstrual bleeding.
At the same time, the decline in ovarian hormones removes some of the feedback that suppresses FSH production. FSH can then begin rising again, helping initiate development of follicles for the next cycle.
This is how one menstrual cycle transitions into the next.
What happens if pregnancy occurs?
If an egg is fertilized and the resulting embryo implants in the uterine lining, hormonal signals from the developing pregnancy help preserve the corpus luteum.
The corpus luteum continues producing progesterone, which helps maintain the uterine lining and supports the early stages of pregnancy. Human chorionic gonadotropin, or hCG, is produced after implantation and helps maintain this hormonal support during early pregnancy.
As pregnancy progresses, the placenta takes over much of the production of progesterone and estrogen.
When menstrual symptoms may need medical attention
Some variation in menstrual cycles is normal, but unusually severe, persistent, or disruptive symptoms can signal an underlying problem.
Medical evaluation is appropriate for symptoms such as very heavy menstrual bleeding, severe or worsening pelvic pain, bleeding between periods, bleeding after sex, consistently very irregular periods, or periods that stop unexpectedly when pregnancy is not the explanation. Severe symptoms that repeatedly interfere with school, work, sleep, or normal activities also deserve attention.
Conditions such as endometriosis, uterine fibroids, thyroid disorders, hormonal disorders, and other reproductive health problems can affect menstrual bleeding or pain. The menstrual cycle itself is normal biology, but symptoms that are unusually intense or disruptive should not simply be dismissed as something everyone has to endure.
The menstrual cycle is ultimately a coordinated process controlled by changing hormone signals. The ovaries prepare and release an egg, the uterus prepares for a possible pregnancy, and, when pregnancy does not occur, hormone levels fall and the uterine lining is shed. Understanding these phases makes it easier to see why menstrual bleeding, ovulation-related changes, and premenstrual symptoms occur—and why the timing and intensity of those changes can differ from one person to another.
