Hormonal changes during the menstrual cycle coordinate the growth of an egg-containing follicle, the release of an egg, and changes in the uterus that prepare it for a possible pregnancy. The main hormones involved are follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone. Their levels rise and fall in a coordinated pattern, and those changes drive the major events of the cycle.
The cycle is not simply a sequence of four equal hormonal stages. Instead, hormones continuously influence one another through feedback signals between the brain, pituitary gland, ovaries, and uterus. The timing and magnitude of these changes can vary from one person and one cycle to another.
How the hormonal cycle is controlled
The hormonal cycle begins with communication between the brain and the reproductive system. A region of the brain called the hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses. GnRH stimulates the pituitary gland to release FSH and LH.
FSH and LH act mainly on the ovaries. FSH helps ovarian follicles grow, while LH contributes to follicle development, ovulation, and the hormonal changes that follow ovulation.
As follicles develop, they produce estrogen, particularly estradiol. After ovulation, the follicle that released the egg changes into a temporary hormone-producing structure called the corpus luteum, which produces substantial amounts of progesterone as well as estrogen.
These hormones then feed information back to the hypothalamus and pituitary gland. Depending on their concentration and the stage of the cycle, estrogen, progesterone, and other ovarian signals can either suppress or stimulate the release of FSH and LH.
This feedback system is what gives the menstrual cycle its characteristic hormonal pattern.
What happens to hormones during menstruation
Menstruation occurs when estrogen and progesterone levels fall after the previous cycle has not resulted in pregnancy.
During the second half of the previous cycle, the corpus luteum produces progesterone and estrogen. If pregnancy does not occur, the corpus luteum eventually breaks down. As it loses its ability to produce these hormones, progesterone and estrogen levels decline.
The drop in these hormones causes the uterine lining, or endometrium, to break down and be shed through the vagina. This is menstrual bleeding.
At the same time, the loss of hormonal feedback allows FSH to begin increasing. The rise in FSH helps recruit a group of ovarian follicles for the next cycle.
Menstruation therefore overlaps with the beginning of a new ovarian cycle. While the uterus is shedding its previous lining, follicles in the ovaries are already beginning to develop under the influence of FSH.
The follicular phase: estrogen gradually rises
The follicular phase begins on the first day of menstruation and continues until ovulation. Its length can vary considerably, making it a major source of variation in overall cycle length.
Early in this phase, several follicles in the ovaries respond to FSH. Each follicle contains an immature egg. As the follicles grow, their hormone production changes.
One follicle typically becomes the dominant follicle and continues developing while the others stop progressing. The developing follicle produces increasing amounts of estrogen.
As estrogen rises, it stimulates the growth and thickening of the uterine lining. The uterus is effectively rebuilding the tissue that was shed during menstruation.
For much of the follicular phase, estrogen also exerts negative feedback on the hypothalamus and pituitary. This helps reduce FSH levels. The decline in FSH contributes to the selection of a dominant follicle because the developing follicles differ in how well they can continue growing as FSH becomes less abundant.
Why estrogen eventually triggers an LH surge
One of the most important changes in the menstrual cycle occurs near ovulation.
For much of the cycle, estrogen suppresses FSH and LH secretion through negative feedback. But when the dominant follicle produces sufficiently high estrogen for a sustained period, the feedback relationship changes. High estrogen instead produces positive feedback on the hypothalamus and pituitary.
This causes a rapid increase in LH known as the LH surge. FSH also rises, although its increase is smaller.
The LH surge triggers a series of changes in the mature follicle that culminate in ovulation, when the follicle releases the egg.
Ovulation is therefore not caused simply by estrogen reaching a particular level. It results from the interaction between sustained high estrogen and the brain-pituitary hormonal feedback system.
Ovulation: the hormonal turning point
Ovulation marks the transition between the follicular and luteal phases.
The LH surge causes the mature follicle to undergo changes that prepare it to release the egg. The follicle eventually ruptures, releasing the egg from the ovary.
Around this time, estrogen levels have reached a high point and then begin to change. LH also falls after the surge.
The follicle does not simply disappear after releasing the egg. Its remaining cells reorganize into the corpus luteum, which becomes an important source of progesterone during the luteal phase.
The luteal phase: progesterone becomes dominant
After ovulation, the luteal phase begins. The corpus luteum produces progesterone and also continues producing estrogen.
Progesterone has a major effect on the uterus. It changes the previously thickened endometrium into a state that is more suitable for supporting a potential pregnancy. The uterine lining becomes more glandular and functionally prepared to respond to an implanted embryo.
Progesterone also suppresses the hypothalamus and pituitary, reducing stimulation of FSH and LH. This helps prevent another dominant follicle from developing and another ovulation from occurring during the same cycle.
The corpus luteum normally remains active for only a limited time if pregnancy does not occur.
What happens when pregnancy does not occur
If the egg is not fertilized and pregnancy does not develop, the corpus luteum eventually loses its function.
As progesterone and estrogen levels fall, the hormonal support for the uterine lining disappears. Blood flow and tissue structure within the endometrium change, leading to the breakdown and shedding of the lining.
The resulting menstrual bleeding marks the beginning of the next cycle.
The declining estrogen and progesterone also remove some of the negative feedback on the hypothalamus and pituitary. FSH can then begin rising again, helping initiate development of follicles for the next cycle.
The cycle is therefore continuous: the hormonal decline that ends one cycle helps create the conditions that begin the next one.
What changes if pregnancy occurs
If pregnancy develops, the hormonal pattern changes before the corpus luteum would normally disappear.
Early pregnancy produces human chorionic gonadotropin (hCG), a hormone that supports the corpus luteum. Continued corpus luteum activity maintains progesterone production, along with estrogen production, helping maintain the uterine lining.
Because progesterone and estrogen remain elevated rather than falling as they would at the end of a nonpregnant cycle, the usual hormonal events that lead to menstruation do not occur.
Later in pregnancy, the placenta becomes an important source of hormones that support the pregnancy.
How the major hormones change across the cycle
The four hormones most closely associated with the menstrual cycle have different jobs and different patterns.
FSH generally rises early in the cycle and helps stimulate follicle development. Its levels are then suppressed as ovarian hormones increase, although it participates in the smaller hormonal changes surrounding ovulation.
LH remains relatively low for much of the cycle before undergoing a sharp surge that triggers ovulation. After ovulation, LH activity helps support the corpus luteum.
Estrogen rises as the dominant follicle develops. It stimulates growth of the uterine lining and, when sustained at high levels, contributes to the LH surge. Estrogen then changes during the luteal phase before falling if pregnancy does not occur.
Progesterone is relatively low before ovulation but rises substantially afterward as the corpus luteum becomes active. It prepares and maintains the uterine lining during the luteal phase. If pregnancy does not occur, progesterone falls as the corpus luteum breaks down.
These hormones do not operate independently. The significance of any one hormone depends partly on what the other hormones are doing and where the person is in the cycle.
Why hormonal changes can affect how someone feels
The rise and fall of reproductive hormones can influence tissues throughout the body, not only the ovaries and uterus. Changes in estrogen and progesterone can affect the brain and other physiological systems, which helps explain why some people notice changes in symptoms or body function at different points in the cycle.
Some people experience breast tenderness, bloating, changes in appetite, headaches, fatigue, mood changes, or other symptoms before menstruation. These symptoms vary widely. Some people experience few noticeable changes, while others experience substantial symptoms.
Premenstrual symptoms are particularly associated with the hormonal changes of the luteal phase and the transition toward menstruation. They are not simply caused by having a high or low level of one hormone in isolation; individual sensitivity to changing hormone levels and interactions with other biological systems also matter.
Why menstrual cycles do not always follow the same pattern
Hormonal patterns are coordinated, but they are not identical from cycle to cycle.
The timing of ovulation can vary, and the follicular phase is generally more variable in length 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 reproductive hormone patterns.
A cycle that differs somewhat from the previous one does not necessarily mean that something is wrong. However, major or persistent changes in bleeding patterns, cycle timing, or symptoms can sometimes reflect an underlying hormonal or reproductive problem.
Understanding the hormonal sequence helps explain why the menstrual cycle is best viewed as a dynamic system rather than a fixed schedule. FSH helps recruit and develop follicles, rising estrogen supports the uterine lining and eventually triggers the LH surge, the LH surge leads to ovulation, and progesterone becomes prominent afterward. When pregnancy does not occur, the corpus luteum breaks down, estrogen and progesterone fall, and menstruation begins again.