The menstrual cycle has four phases: menstruation, the follicular phase, ovulation, and the luteal phase. A typical cycle lasts 21 to 35 days in adults, with 28 days as the average. Each phase is driven by shifting levels of hormones that prepare the body for a potential pregnancy, then reset when pregnancy doesn’t occur.
Menstruation (Days 1 to 5)
Menstruation is the shedding of the uterine lining, and day one of your period marks day one of your cycle. The lining breaks down because progesterone and estrogen levels have dropped, signaling that pregnancy didn’t happen. The usual amount of blood lost during an entire period is 10 to 35 milliliters, roughly one to two tablespoons. Anything over 80 milliliters (soaking through about 16 regular pads or tampons across the whole period) is considered very heavy flow.
Most periods last between three and seven days. During this time, some people also experience cramping, bloating, headaches, or fatigue. These symptoms are largely caused by hormone-like compounds that help the uterus contract and shed its lining.
The Follicular Phase (Days 1 to 13)
The follicular phase overlaps with menstruation. It starts on day one and lasts until ovulation. During this stretch, the brain releases a hormone called FSH (follicle-stimulating hormone), which prompts several small fluid-filled sacs called follicles to start growing in the ovaries. Each follicle contains an immature egg.
As these follicles grow, one of them pulls ahead. The “winning” follicle produces enough estrogen to suppress FSH levels through a feedback loop, essentially cutting off the hormonal supply that the smaller follicles need to keep developing. Those less mature follicles die off, and the dominant follicle continues to grow. This is how your body selects a single egg for ovulation in most cycles.
Rising estrogen also does something you can see and feel. It thickens the uterine lining (endometrium) from its post-period baseline up to about 12 to 13 millimeters by the time ovulation approaches. Cervical mucus changes too. Early in the follicular phase, mucus is dry or sticky and paste-like, often white or light yellow. As estrogen climbs, it gradually becomes creamy and smooth, then transitions to wet and watery. These visible shifts give a real-time signal of where you are in your cycle.
Ovulation (Around Day 14)
Ovulation is when the dominant follicle releases its mature egg into the fallopian tube. It’s the shortest phase, lasting roughly 24 hours, though the hormonal events that trigger it begin a day or two earlier.
The trigger is a sharp spike in luteinizing hormone, commonly called the LH surge. This surge typically begins about 36 hours before the egg is released, with the LH peak occurring about 10 to 12 hours before ovulation. Home ovulation test kits detect this surge in urine, which is why they can give you advance notice that ovulation is approaching.
Estrogen reaches its highest point just before the LH surge, then drops once ovulation occurs. At this moment, cervical mucus is at its most fertile: clear, slippery, and stretchy, often compared to raw egg whites. This consistency helps sperm travel more easily. After ovulation, mucus quickly returns to thick and dry.
Your basal body temperature (your lowest resting temperature, taken first thing in the morning) also shifts after ovulation, rising by less than half a degree Fahrenheit (about 0.3°C). This increase is subtle, so it only shows up if you’ve been charting your temperature daily. Importantly, the temperature rise confirms ovulation has already happened. It doesn’t predict it in advance the way an LH test does.
The Luteal Phase (Days 15 to 28)
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. Its primary job is producing progesterone, the hormone that dominates the second half of your cycle. Progesterone stops the uterine lining from continuing to grow and instead converts it into a spongy, nutrient-rich environment that could support a fertilized egg. During this secretory phase, the endometrium thickens further to about 16 to 18 millimeters.
Progesterone levels during the mid-luteal phase fluctuate dramatically, sometimes shifting eightfold within a 90-minute window. This is normal. It’s also why a single blood draw for progesterone can look very different depending on timing. The luteal phase lasts about 14 days and is the most consistent part of the cycle from person to person. When cycles vary in length, it’s almost always because the follicular phase was shorter or longer, not because the luteal phase changed.
If the egg isn’t fertilized, the corpus luteum breaks down after about 10 to 14 days. Progesterone and estrogen levels fall sharply, and the thickened uterine lining can no longer sustain itself. That triggers menstruation, and the cycle starts over. If the egg is fertilized and implants, the corpus luteum keeps producing progesterone to support the early pregnancy until the placenta takes over.
What the Hormones Are Actually Doing
Four hormones run the cycle, and they work in a relay. FSH kicks things off in the follicular phase by stimulating follicle growth. Estrogen, produced by the growing follicles, builds the uterine lining and eventually triggers the LH surge. LH causes ovulation. Then progesterone, from the corpus luteum, maintains the lining and keeps the body in a holding pattern while it “waits” to see if implantation occurs.
These hormones don’t just affect your reproductive organs. Estrogen influences mood, energy, and skin. Progesterone can cause bloating, breast tenderness, and sleepiness. The premenstrual symptoms many people experience in the last week of the luteal phase are a direct result of progesterone peaking and then rapidly declining.
Tracking Your Cycle
You don’t need lab tests to get a general sense of where you are in your cycle. Cervical mucus gives the most accessible day-to-day clue. Dry or pasty mucus means you’re likely in the early follicular phase or post-ovulation. Wet, slippery, egg-white mucus signals your most fertile window, the days leading up to and including ovulation.
Basal body temperature tracking works best in hindsight. Once you see a sustained temperature rise of about 0.3°C over several mornings, you can confirm that ovulation occurred. Combining both methods gives a fuller picture than either one alone.
Cycle length varies more than most people realize. In teens, cycles can range anywhere from 21 to 45 days as the hormonal system matures. For most adults, 21 to 35 days is considered normal. If your cycle falls outside that range consistently, or if the length swings unpredictably by more than a week or two from month to month, that pattern is worth paying attention to.