The menstrual cycle is your body’s monthly process of preparing for pregnancy. Each month, hormones signal the uterus to build up a thick, blood-rich lining. If a fertilized egg doesn’t implant, that lining sheds and exits through the vagina as a period. A normal cycle lasts anywhere from 21 to 35 days, counted from the first day of one period to the first day of the next, and bleeding itself typically lasts 2 to 7 days.
The Four Phases of the Cycle
The menstrual cycle isn’t one continuous event. It moves through four distinct phases, each driven by shifting hormone levels.
Menstruation
This is your period. The upper layer of the uterine lining, called the functional layer, breaks down and flows out along with blood from small vessels in the uterus. The deeper base layer stays intact and serves as the foundation for rebuilding. Most periods last between 2 and 7 days, and total blood loss during a normal period is under 60 milliliters, roughly 4 tablespoons. Losses above 80 milliliters are considered heavy bleeding.
The Follicular Phase
This phase overlaps with menstruation, starting on day one of your period and lasting about 13 to 14 days. Hormone signals from the brain prompt several small fluid-filled sacs (follicles) to start growing on the surface of the ovaries. Usually only one follicle matures fully and develops a viable egg. At the same time, rising estrogen levels stimulate the uterine lining to start thickening again.
Ovulation
Ovulation is the release of a mature egg from one ovary. It typically happens about 14 days before your next period starts. A massive surge of luteinizing hormone (LH), lasting 36 to 48 hours, triggers enzymes that break down the follicle wall and release the egg within about 16 to 32 hours. Once released, the egg survives for only 12 to 24 hours.
The Luteal Phase
After the egg is released, it travels through the fallopian tube toward the uterus. The empty follicle transforms into a temporary structure that pumps out progesterone, peaking about 6 to 8 days after ovulation. Progesterone makes the uterine lining spongy and rich with nutrients, ideal for a fertilized egg to implant. It also raises your basal body temperature by about half a degree Celsius. If no implantation occurs, progesterone and estrogen levels drop, the blood supply to the lining is cut off, and the tissue breaks down. That triggers your next period, and the cycle starts over.
The Hormones Running the Show
Four hormones coordinate the entire cycle. Two come from the brain, and two come from the ovaries.
Follicle-stimulating hormone (FSH) rises early in the cycle to kick-start follicle growth in the ovaries. As the follicles develop, they produce increasing amounts of estrogen, which thickens the uterine lining and eventually triggers the LH surge that causes ovulation. The relationship between estrogen and LH is unusual: for most of the cycle, estrogen actually suppresses LH release, but when estrogen climbs high enough in the late follicular phase, it flips to a positive signal and causes that dramatic LH spike.
After ovulation, progesterone takes over as the dominant hormone. It transforms the lining from a growing state into a receptive one, ready for implantation. If pregnancy doesn’t happen, the structure producing progesterone breaks down, hormone levels fall, and the lining sheds. That drop in both estrogen and progesterone is also what triggers many premenstrual symptoms.
The Fertile Window
Even though ovulation lasts only a day, the window for getting pregnant is about 6 days per cycle. That’s because sperm can survive inside the body for up to 5 days, while the egg is viable for 12 to 24 hours after release. Conception can result from sex that happens up to 5 days before ovulation or 1 day after. The most fertile days are generally the 2 to 3 days leading up to ovulation, when sperm are already in position as the egg is released.
What’s Normal and What’s Not
Cycles between 21 and 35 days are considered normal, and some variation from month to month is expected, especially during the first few years of menstruating and again in the years leading up to menopause. Periods that consistently fall outside that range are worth paying attention to.
Signs that something may need evaluation include:
- Cycles shorter than 21 days or longer than 45 days apart
- A single gap of more than 90 days between periods
- Periods that were regular and then became unpredictable
- Bleeding that lasts longer than 7 days
- Soaking through a pad or tampon more than once every 1 to 2 hours
- Feeling dizzy or faint from blood loss
Not having started a period by age 15, or within 3 years of breast development beginning, can also signal a hormonal or structural issue worth investigating.
Premenstrual Symptoms
In the days before a period, the drop in progesterone and estrogen can produce a wide range of physical and emotional effects. Common physical symptoms include bloating, breast tenderness, headaches, fatigue, joint or muscle pain, acne flare-ups, and changes in digestion like constipation or diarrhea. Weight can temporarily increase due to fluid retention.
Emotional symptoms are just as common. Irritability, mood swings, anxiety, depressed mood, food cravings, difficulty concentrating, and trouble sleeping all fall under the umbrella of premenstrual syndrome (PMS). These symptoms typically start in the luteal phase and resolve within a few days of your period beginning. Not everyone experiences them to the same degree, and their intensity can shift across different stages of life.
When Cycles Begin and End
The first period, called menarche, arrives at a median age of about 11 years and 10 months in the United States. That age has shifted slightly earlier over recent decades. By age 12, roughly half of girls have started menstruating, and by age 14, about 90% have. Cycles in the first year or two are often irregular as the hormonal feedback system matures.
Menstrual cycles continue until menopause, which occurs on average around age 51. In the years leading up to menopause, a transitional phase called perimenopause brings increasingly irregular cycles, changes in flow, and fluctuating hormone levels. Menopause is officially defined as 12 consecutive months without a period.