The menstrual cycle is a repeating sequence of hormonal shifts that prepares the body for pregnancy each month. A typical cycle lasts about 28 days, but anything from 21 to 35 days is normal. The process involves a continuous conversation between the brain and the ovaries, with four hormones driving changes in the ovaries and uterus simultaneously.
The Brain Runs the Show
The cycle is controlled by a feedback loop between three structures: a small region at the base of the brain (the hypothalamus), the pituitary gland just below it, and the ovaries. The hypothalamus sends a signal to the pituitary, which releases two key hormones into the bloodstream: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These travel to the ovaries and tell them what to do. The ovaries, in turn, produce estrogen and progesterone, which signal back to the brain to either ramp up or dial down production. This back-and-forth keeps the entire cycle on schedule.
Phase 1: Menstruation
Day 1 of your cycle is the first day of your period. This is when the uterus sheds its inner lining (the endometrium) because no pregnancy occurred in the previous cycle. Bleeding typically lasts 2 to 7 days. Blood loss above 80 milliliters per cycle is considered heavy, though most people lose considerably less than that.
During menstruation, estrogen and progesterone are both at their lowest levels. This hormonal low point is what triggered the shedding in the first place: without progesterone to maintain it, the thickened uterine lining breaks down and exits through the vagina.
Phase 2: The Follicular Phase
The follicular phase overlaps with menstruation, starting on day 1 and lasting until ovulation. As your period winds down, FSH from the pituitary gland stimulates several small fluid-filled sacs (follicles) inside the ovaries to start growing. Each follicle contains an immature egg. Over the course of about two weeks, one follicle outpaces the others and becomes dominant, while the rest stop developing.
As the dominant follicle grows, it produces increasing amounts of estrogen. This rising estrogen does two things. First, it tells the uterine lining to start rebuilding, growing thicker with new blood vessels and tissue to create a potential home for a fertilized egg. Second, it signals back to the brain, eventually triggering the next phase.
Many people notice they feel more energetic during the mid-to-late follicular phase, after their period ends. Estrogen has mood-boosting effects, and the body is essentially in building mode.
Phase 3: Ovulation
Ovulation happens around day 14 in a 28-day cycle, though the exact timing varies. When estrogen from the dominant follicle reaches a critical threshold, it triggers a sudden surge of LH from the pituitary gland. This LH surge is the direct cause of ovulation: within about 24 to 36 hours, the dominant follicle ruptures and releases a mature egg into the fallopian tube.
The released egg is viable for roughly 12 to 24 hours. If sperm are present in the fallopian tube during this window, fertilization can occur. This is the shortest phase of the cycle, sometimes described as a single event rather than a true phase. Home ovulation test kits work by detecting that LH surge in urine.
Phase 4: The Luteal Phase
After the egg is released, the empty follicle left behind in the ovary transforms into a temporary structure called the corpus luteum. This small mass of cells has one critical job: producing progesterone.
Progesterone changes the character of the uterine lining. While estrogen built it up during the follicular phase, progesterone now makes it spongy, rich in nutrients, and receptive to a fertilized egg. It also slightly raises body temperature, which is why tracking morning temperature can help identify when ovulation has occurred.
The luteal phase lasts from roughly day 15 to day 28 and is more consistent in length than the follicular phase. If the egg is fertilized and implants in the uterine wall, the corpus luteum keeps producing progesterone for about 12 weeks until the placenta takes over. If fertilization doesn’t happen, the corpus luteum begins to break down around 10 days after ovulation. As it degrades, progesterone and estrogen levels plummet. Without progesterone to sustain it, the uterine lining can no longer maintain itself. It sheds, your period begins, and the cycle starts over.
Why the Luteal Phase Affects How You Feel
The drop in progesterone and estrogen in the week or two before your period is what drives premenstrual symptoms. Common physical changes include bloating, breast tenderness, headaches, cramps, and joint or muscle pain. Mood-related symptoms like irritability, anxiety, sadness, food cravings, trouble sleeping, and difficulty concentrating are also tied to this hormonal decline. For most people, these symptoms clear up within two to three days after bleeding starts, as the body adjusts to its new hormonal baseline.
A small percentage of people experience a severe form of premenstrual symptoms called PMDD (premenstrual dysphoric disorder), which involves intense mood disturbances like lasting anger, panic attacks, or feelings of despair that significantly interfere with daily life. PMDD follows the same hormonal trigger but produces a much stronger response.
What Affects Cycle Regularity
Because the entire cycle depends on precise hormonal signaling between the brain and ovaries, anything that disrupts that communication can shift your timing. Stress is one of the most common culprits: the hypothalamus, which initiates the cycle, also responds to stress hormones, and sustained stress can delay or skip ovulation entirely. Significant weight changes in either direction affect estrogen levels, since fat tissue plays a role in estrogen production. Intense exercise routines that result in very low body fat, common among long-distance runners, dancers, and gymnasts, can suppress the cycle altogether.
Starting or changing hormonal birth control resets the cycle’s natural signaling, and it can take a few months for regularity to return after stopping. Travel, shift work, illness, and disruptions to your daily routine can all introduce temporary irregularity. Gradual weight management, moderate exercise, and stress reduction techniques tend to support more predictable cycles over time.
The Bigger Timeline
Menstrual cycles begin at puberty and continue until menopause. Most people experience menopause between ages 45 and 55, when the ovaries stop releasing eggs and estrogen production drops permanently. Between those endpoints, the cycle repeats roughly 400 to 500 times over a reproductive lifetime, with each cycle running through the same sequence of follicle growth, ovulation, and either pregnancy or shedding. Cycles are often irregular in the first few years after they start and again in the years leading up to menopause, as hormone levels fluctuate more unpredictably during those transitions.