The menstrual cycle has four distinct phases: the menstrual phase, the follicular phase, ovulation, and the luteal phase. In a typical 28-day cycle, these phases overlap and flow into one another, driven by shifting levels of hormones produced by the brain and ovaries. Understanding what happens in each phase helps explain everything from period cramps to fertility windows to mood shifts throughout the month.
The Menstrual Phase (Days 1 to 5)
Day 1 of your cycle is the first day of your period. This phase begins because the hormones estrogen and progesterone both drop sharply at the end of the previous cycle. Without those hormones to sustain it, the thickened lining of the uterus breaks down and sheds through the vagina. That shedding is your period.
The cramping you feel during menstruation is your uterus physically contracting to push out that lining. A typical period lasts anywhere from two to seven days, and the flow is a mix of blood, tissue, and mucus. Cervical mucus during and just after your period tends to be dry or tacky, often white or slightly yellow-tinged. By the end of this phase, hormone levels are at their lowest point in the entire cycle, which is what signals the brain to kick off the next phase.
The Follicular Phase (Days 1 to 13)
The follicular phase actually overlaps with menstruation, starting on day 1 and continuing until ovulation. Once hormone levels bottom out, a small gland at the base of your brain (the pituitary gland) releases follicle-stimulating hormone, or FSH. This hormone does exactly what its name suggests: it activates the ovaries to start developing follicles, which are tiny fluid-filled sacs that each contain an immature egg.
Several follicles begin growing at once, but usually only one becomes dominant. That dominant follicle pumps out increasing amounts of estrogen as it matures. Rising estrogen does two important things. First, it thickens the uterine lining (called the endometrium) in preparation for a potential pregnancy. The lining grows from nearly nothing after your period to about 12 to 13 millimeters thick before ovulation. Second, the rising estrogen eventually triggers the hormonal surge that causes ovulation.
You can actually observe this phase through changes in cervical mucus. Around days 4 to 6, mucus is sticky and slightly damp. By days 7 to 9, it becomes creamier and wetter. As you approach ovulation around days 10 to 14, the mucus becomes stretchy, slippery, and resembles raw egg whites. That slippery consistency makes it easier for sperm to travel through the reproductive tract.
Ovulation (Around Day 14)
Ovulation is the shortest phase, lasting roughly 24 hours. It’s triggered when estrogen levels climb high enough to flip a switch in the brain’s signaling system. Throughout most of the cycle, estrogen actually suppresses the brain’s release of reproductive hormones. But at a certain concentration, it does the opposite: it amplifies the brain’s response and causes a massive burst of luteinizing hormone, or LH. This reversal is what makes the menstrual cycle cyclical rather than static.
About 36 to 40 hours after LH surges in the blood, the dominant follicle ruptures and releases a mature egg into the fallopian tube. Home ovulation test kits detect LH in urine, and once that positive result appears, ovulation typically follows within 12 to 24 hours. The released egg is viable for about 12 to 24 hours. Since sperm can survive in the reproductive tract for up to five days, the total fertile window extends from roughly five days before ovulation through one day after.
Your basal body temperature (your resting temperature first thing in the morning) rises slightly after ovulation, typically by less than half a degree Fahrenheit. This small shift, caused by progesterone, stays elevated for the rest of the cycle and is one way to confirm that ovulation has already occurred.
The Luteal Phase (Days 15 to 28)
After the egg leaves the follicle, the empty sac doesn’t just disappear. It transforms into a temporary structure called the corpus luteum, a yellowish mass of cells that forms right where the follicle used to be. The corpus luteum’s primary job is producing progesterone, the hormone that prepares the uterine lining to receive a fertilized egg. Under progesterone’s influence, the lining continues to thicken and becomes rich with blood vessels and nutrients, reaching 16 to 18 millimeters thick before your period.
Progesterone also works with estrogen to slow down the brain’s release of FSH and LH during this phase. That feedback loop prevents new follicles from developing while the body waits to see if pregnancy occurs. Cervical mucus dries up noticeably during the luteal phase, becoming thick or nearly absent.
If a fertilized egg implants in the uterine lining, it releases a hormone that tells the corpus luteum to keep producing progesterone, sustaining the pregnancy. If implantation doesn’t happen, the corpus luteum breaks down after about 10 to 14 days. Progesterone and estrogen levels plummet, the uterine lining loses its hormonal support, and menstruation begins. The cycle starts over.
How the Phases Connect
The four phases aren’t independent events. They’re linked by a feedback system between the brain and the ovaries. Low hormones at the start of the cycle tell the brain to release FSH. Rising estrogen from growing follicles eventually triggers the LH surge that causes ovulation. High progesterone after ovulation suppresses the brain’s signals. And when progesterone drops, the brain ramps FSH back up again.
This is why cycle length varies between people and even between months. The follicular phase is the most variable part. Some people ovulate on day 10, others on day 20. The luteal phase, by contrast, is relatively consistent at 10 to 14 days. So if your cycle is longer or shorter than 28 days, the difference is almost always in the first half.
Tracking Your Cycle Phase
You don’t need lab tests to get a rough sense of where you are in your cycle. Cervical mucus is one of the most accessible indicators. Dry or tacky mucus suggests the early follicular phase or the luteal phase. Creamy, wet mucus signals mid-follicular. Stretchy, egg-white mucus means you’re approaching or at ovulation, your most fertile window.
Basal body temperature adds another data point, but it only confirms ovulation after the fact. You’ll see temperatures stay elevated for the rest of the cycle once the luteal phase begins. Combining mucus tracking with temperature can give you a fairly reliable picture of your cycle pattern over a few months. Many period-tracking apps use these inputs alongside cycle length data to estimate phase timing, though predictions are more accurate once you’ve logged several cycles.