A period happens when your uterus sheds its inner lining because no pregnancy occurred that month. The whole process is driven by a repeating cycle of hormones that build up the lining, release an egg, and then trigger the lining to break down if the egg goes unfertilized. A typical cycle runs 21 to 35 days, with the average around 28 days.
The Four Hormones Running the Show
Your menstrual cycle is controlled by four hormones working in sequence. Two come from a small gland at the base of your brain (the pituitary), and two come from your ovaries.
The brain hormones are FSH (follicle-stimulating hormone) and LH (luteinizing hormone). FSH kicks things off at the start of each cycle by telling your ovaries to start growing small fluid-filled sacs called follicles, each containing an immature egg. As those follicles grow, they produce estrogen, the first ovarian hormone. Estrogen’s main job is thickening the uterine lining with blood and tissue so it could support a pregnancy. When estrogen climbs high enough, usually around day 13, it triggers a sudden spike in LH. That LH surge is the signal for ovulation: the dominant follicle ruptures and releases a mature egg.
After the egg is released, the empty follicle transforms into a structure called the corpus luteum, which pumps out progesterone, the fourth hormone. Progesterone stabilizes the thickened lining and makes it receptive to a fertilized egg. If no fertilized egg implants, the corpus luteum breaks down, progesterone and estrogen both drop sharply, and that drop is what causes the lining to shed. That shedding is your period.
What Happens in Each Phase
The Follicular Phase
This phase starts on day one of your period and lasts about 13 to 14 days on average, though it varies more than any other phase. During this time, several follicles in your ovaries begin developing, but typically only one becomes dominant. Meanwhile, estrogen is steadily rising, and your uterine lining is rebuilding from scratch after the last period.
Ovulation
Ovulation itself is surprisingly brief, lasting only 16 to 32 hours. The LH surge triggers the dominant follicle to rupture and release its egg into the fallopian tube. The egg survives for about 12 to 24 hours after release. This is the only window in which fertilization can happen, though sperm can survive in the reproductive tract for several days beforehand.
The Luteal Phase
After ovulation, the luteal phase lasts a fairly consistent 14 days. Progesterone is the dominant hormone here, keeping the uterine lining thick and nutrient-rich. If the egg isn’t fertilized, the corpus luteum degrades, hormone levels fall, and the cycle resets with menstruation.
What Menstrual Fluid Actually Contains
Period fluid isn’t pure blood. It’s a mix of arterial and venous blood, shed endometrial tissue, cervical mucus, vaginal secretions, and even immune cells and stem cells. It also contains water, proteins, lipids, hormones, and microorganisms from the vaginal microbiome. This is why menstrual fluid looks and behaves differently from blood you’d see from a cut: it’s often darker, thicker, and doesn’t clot the same way.
The total volume lost during an average period is roughly 30 to 40 milliliters, or about two to three tablespoons. Anything above 80 milliliters is considered heavy bleeding. Most periods last between three and seven days.
Why PMS Happens
Premenstrual symptoms hit in the luteal phase, the week or two before your period, when progesterone is high and then dropping. The exact cause isn’t fully pinned down, but two factors play the biggest roles. First, the rapid hormonal shifts themselves seem to affect the body directly, causing bloating, breast tenderness, acne flare-ups, fatigue, and joint or muscle pain. Second, those same hormonal changes affect serotonin, a brain chemical tied to mood regulation. Dips in serotonin can contribute to the irritability, food cravings, trouble sleeping, and low mood that many people experience before their period.
PMS symptoms disappear once the period starts and hormone levels stabilize at their lowest point. Not everyone experiences PMS with the same intensity. For some people it’s barely noticeable; for others it significantly disrupts daily life.
When Periods Start and What’s Normal
Most people get their first period around age 11 to 12. CDC data shows the median age has shifted slightly earlier over recent decades, from 12.1 years in 1995 to 11.9 years by 2013 to 2017. In the first couple of years, cycles are often irregular and can range from 21 to 45 days. By adulthood, cycles typically settle into a 21-to-35-day range.
Cycle length can still vary from month to month based on stress, sleep, weight changes, exercise, and illness. A cycle that’s consistently within that 21-to-35-day window is considered normal even if it doesn’t land on exactly 28 days every time.
Signs Something May Be Off
Some period symptoms are worth paying attention to. Needing to change your pad or tampon every one to two hours, or needing to double up on products (like a pad and tampon together), suggests unusually heavy bleeding. Period pain that’s severe enough to interfere with your daily routine, especially if it gets worse over time, can signal conditions like endometriosis. Periods that suddenly stop for several months when you’re not pregnant, or cycles that are consistently shorter than 21 days or longer than 35 days, are also worth discussing with a doctor.