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 reproductive hormones, and it includes the development of an egg, preparation of the uterine lining, ovulation, and—if pregnancy does not occur—menstruation.
Although the cycle is often described as a 28-day process, that is only an average. Cycle length and the timing of individual phases can vary considerably from person to person and from one cycle to another.
What happens during the menstrual cycle?
The menstrual cycle begins on the first day of menstrual bleeding and continues until the day before the next period begins. It can be divided into four overlapping phases: menstruation, the follicular phase, ovulation, and the luteal phase.
These phases are controlled by changing levels of hormones, particularly estrogen and progesterone, along with follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
The overall purpose is to prepare an egg for possible fertilization and prepare the uterus to support a pregnancy if fertilization occurs.
Menstruation: when the uterine lining is shed
Menstruation is the bleeding that occurs when pregnancy has not occurred.
During the previous cycle, hormones caused the lining of the uterus, called the endometrium, to become thicker and more suitable for supporting an embryo. If an egg is not successfully fertilized and implanted, hormone levels fall, particularly progesterone and estrogen. This hormonal change causes the uterine lining to break down and leave the body through the vagina.
The first day of menstrual bleeding is considered day 1 of the menstrual cycle.
Menstrual blood is not simply blood. It contains blood, mucus, vaginal secretions, and tissue from the uterine lining. The amount and appearance of menstrual flow can vary between individuals and from one cycle to another.
The follicular phase: an egg begins to mature
The follicular phase begins on the first day of menstruation and continues until ovulation.
During this phase, the brain and ovaries communicate through hormones. The pituitary gland releases follicle-stimulating hormone (FSH), which encourages groups of small structures called ovarian follicles to grow. Each follicle contains an immature egg.
As the follicles develop, they produce estrogen. Usually, one follicle becomes dominant and continues developing toward ovulation, while the others stop developing.
Rising estrogen also stimulates changes in the uterine lining. After menstruation has ended, the endometrium gradually becomes thicker again as it prepares for a possible pregnancy.
The follicular phase is the part of the cycle that tends to vary most in length. This is one reason menstrual cycles do not always occur at exactly the same interval.
Ovulation: when an egg is released
Ovulation occurs when a mature follicle releases an egg from an ovary.
As the dominant follicle develops, estrogen levels rise. When estrogen reaches a sufficiently high level for long enough, it triggers a sharp increase in luteinizing hormone (LH). This is known as the LH surge and it causes the mature follicle to release its egg.
The released egg enters the fallopian tube, where it can potentially encounter sperm and become fertilized.
Ovulation does not necessarily happen on day 14 of every menstrual cycle. The familiar 28-day cycle is an average, and the timing of ovulation depends largely on how long the follicular phase lasts.
The egg remains capable of being fertilized for a relatively short period after ovulation. Sperm, however, can survive for several days under favorable conditions in the female reproductive tract. As a result, pregnancy can result from sexual intercourse occurring in the days before ovulation as well as around the time of ovulation.
The luteal phase: the body prepares for possible pregnancy
After ovulation, the follicle that released the egg changes into a temporary hormone-producing structure called the corpus luteum.
The corpus luteum produces substantial amounts of progesterone and also produces estrogen. Progesterone helps maintain the uterine lining in a condition suitable for implantation and early pregnancy.
If the egg is fertilized and the resulting embryo implants in the uterus, hormonal signals associated with pregnancy maintain the corpus luteum for a time so that progesterone production continues.
If fertilization and implantation do not lead to pregnancy, the corpus luteum eventually breaks down. Progesterone and estrogen levels fall, the uterine lining loses hormonal support, and menstruation begins. The next menstrual cycle then starts.
How hormones control the menstrual cycle
The menstrual cycle is coordinated by a communication system involving the hypothalamus, pituitary gland, and ovaries.
The hypothalamus releases gonadotropin-releasing hormone (GnRH), which signals the pituitary gland. The pituitary then releases FSH and LH. These hormones act on the ovaries, where they regulate follicle development, ovulation, and hormone production.
The ovaries produce estrogen and progesterone, which act on the uterus and also provide feedback to the brain and pituitary.
This feedback system allows hormone levels to change in a coordinated sequence rather than independently. A change in one hormone can influence the production or release of others, helping move the cycle from one phase to the next.
How long is a menstrual cycle?
A menstrual cycle is measured from the first day of one period to the first day of the next.
A 28-day cycle is commonly used as a reference point, but normal cycles can be shorter or longer. The length can also change over time, particularly during adolescence, around the transition toward menopause, and in response to factors that affect reproductive hormones.
Importantly, cycle length and menstrual bleeding are not the same thing. Menstrual bleeding usually lasts only part of the overall cycle.
The timing of ovulation can also differ substantially between cycles. Therefore, assuming that every person ovulates on a particular calendar day is inaccurate.
What happens if pregnancy occurs?
If fertilization occurs, the resulting embryo begins developing as it travels toward the uterus. After reaching the uterus, it may implant in the uterine lining.
Pregnancy changes the normal hormonal sequence of the menstrual cycle. Hormonal signals from the developing pregnancy help maintain progesterone production, allowing the uterine lining to remain in place rather than being shed as a menstrual period.
As pregnancy progresses, hormone production shifts increasingly toward the placenta, which supports the pregnancy and produces hormones needed to maintain it.
Because this hormonal system prevents the normal menstrual cycle from proceeding in the usual way, menstruation does not normally occur during pregnancy.
What is normal menstrual bleeding?
Menstrual bleeding differs widely among people. Flow may be lighter at the beginning or end of a period and heavier during other days. Blood can appear bright red, dark red, or brown, particularly as it takes longer to leave the body.
Small clots can occur during menstruation because menstrual blood contains tissue from the uterine lining and may partially clot before leaving the uterus.
Mild cramping is also common. The uterus contracts partly because of substances called prostaglandins, which are involved in inflammation and muscle contraction. These contractions help the uterus shed its lining but can also cause menstrual pain.
Some people experience other symptoms around menstruation, such as bloating, breast tenderness, headaches, fatigue, or changes in mood.
Why do menstrual symptoms change from cycle to cycle?
Hormone levels do not follow exactly the same pattern every month. The timing of follicle development, ovulation, and hormone production can vary, which can affect both the timing and symptoms of menstruation.
Sleep, stress, changes in body weight, illness, intense physical activity, and other factors can also influence menstrual patterns by affecting the hormonal system that regulates the cycle.
Occasional variation is therefore not necessarily a sign that something is wrong. Persistent or major changes, however, can sometimes indicate an underlying reproductive or general health problem.
What is the difference between the menstrual cycle and a period?
The terms are sometimes used interchangeably, but they describe different things.
The menstrual cycle is the entire sequence of hormonal and reproductive changes from the beginning of one period to the beginning of the next.
A period, or menstruation, is only the part of that cycle when the uterine lining is shed and menstrual bleeding occurs.
In other words, menstruation is one phase of the menstrual cycle, not the entire cycle.
When can menstrual cycles become irregular?
Menstrual cycles can become irregular when the hormonal signals controlling ovulation are disrupted or when ovulation does not occur regularly.
Irregular cycles can occur naturally during the first years after menstruation begins and during the years leading up to menopause. They can also be associated with pregnancy, hormonal contraception, significant changes in weight, excessive physical stress, certain medical conditions, or other changes affecting hormone regulation.
A cycle that occasionally arrives earlier or later is different from a persistent pattern of unusually irregular, very frequent, very infrequent, or absent periods.
Changes that are persistent, severe, or accompanied by unusually heavy bleeding, significant pain, or other concerning symptoms warrant discussion with a healthcare professional.
Why understanding the menstrual cycle matters
The menstrual cycle is more than a schedule for periods. It is a coordinated reproductive process in which hormones produced by the brain and ovaries influence the development of an egg, the release of that egg, and the preparation and maintenance of the uterine lining.
Understanding the sequence—from follicle development and rising estrogen to the LH surge, ovulation, progesterone production, and eventual menstruation—makes it easier to understand changes in periods, fertility, menstrual symptoms, and reproductive health.
