The menstrual cycle has four phases: the menstrual phase, the follicular phase, ovulation, and the luteal phase. A full cycle averages about 28 to 29 days, counted from the first day of one period to the day before the next. Each phase is driven by shifting hormone levels that prepare the body for a potential pregnancy, then reset if one doesn’t occur.
Understanding what happens in each phase can help you make sense of the physical changes you notice throughout the month, from energy shifts to changes in discharge to cramps.
The Menstrual Phase (Days 1 to 7)
Your cycle begins on the first day of your period. When no pregnancy has occurred, levels of both estrogen and progesterone drop sharply. That progesterone withdrawal is the actual trigger for menstruation. Without progesterone to maintain it, the thickened uterine lining breaks down. Blood vessels in the lining become fragile and permeable, immune cells flood the area, and enzymes dissolve the tissue so it can be shed.
Most people bleed for three to five days, though anywhere from three to seven days is typical. Estrogen sits at its lowest point at the start of your period, which is one reason many people feel lower energy during the first few days.
The Follicular Phase (Days 1 to 14)
The follicular phase overlaps with menstruation. It starts on day one and lasts until ovulation. During this phase, your brain releases follicle-stimulating hormone (FSH), which signals small fluid-filled sacs called follicles on one of your ovaries to start growing. Each follicle contains an immature egg.
Between days 6 and 14, several follicles begin developing. By days 10 to 14, only one “dominant” follicle wins out and continues maturing into a fully developed egg. The rest stop growing. As these follicles develop, they produce rising amounts of estrogen. By around day 7, blood estrogen levels have climbed significantly, which thickens the uterine lining in preparation for a potential pregnancy.
This rising estrogen is also behind many of the changes you might notice mid-cycle. Energy often increases, mood may lift, and cervical mucus shifts from dry or tacky right after your period to creamy and wet around days 7 to 9, then stretchy and slippery (resembling raw egg whites) around days 10 to 14. That slippery mucus is one of the most reliable external signs that ovulation is approaching.
Ovulation (Around Day 14)
When estrogen reaches a high enough level, it triggers a sudden surge of luteinizing hormone (LH), typically around day 13. This LH surge is what ovulation predictor kits detect in urine. Ovulation itself, the release of a mature egg from the ovary, happens 8 to 20 hours after LH peaks. From the start of the surge to egg release, the whole process takes roughly 24 to 48 hours.
The egg travels into the fallopian tube, where it can be fertilized for about 12 to 24 hours. This is the most fertile window of the cycle, and it includes the days just before ovulation when sperm can already be present and waiting.
Two physical signs mark ovulation. First, cervical mucus reaches its most wet and slippery state. Second, basal body temperature (your temperature at complete rest, taken first thing in the morning) rises slightly, typically less than half a degree Fahrenheit. When that small temperature bump holds steady for three or more days, ovulation has likely already occurred. The temperature shift confirms ovulation after the fact rather than predicting it in advance.
The Luteal Phase (Days 15 to 28)
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. This structure pumps out increasing amounts of progesterone, along with some estrogen. Progesterone is the dominant hormone of the luteal phase, and its job is to prepare the uterine lining for a potential embryo. It causes the lining to thicken further and fill with fluids and nutrients.
The luteal phase lasts approximately 14 days, with a typical range of 12 to 15 days. Unlike the follicular phase, which can vary quite a bit from person to person, the luteal phase tends to be more consistent in length.
If the egg isn’t fertilized, the corpus luteum breaks down, progesterone and estrogen levels fall, and the lining can no longer sustain itself. That drop in hormones triggers menstruation, and the cycle starts over. Cervical mucus dries up during this phase, and many people experience premenstrual symptoms like bloating, breast tenderness, or mood changes as progesterone rises and then drops.
How Much Cycle Length Varies
The 28-day cycle is an average, not a rule. A large Harvard study analyzing over 165,000 cycles found the average length was 28.7 days across the adult population, but individual variation is normal and expected. People under 20 saw their cycle lengths vary by about 5 days on average. Those aged 35 to 39 had the most consistent cycles, varying by about 4 days. After 40, variability increases again, and by age 50 it can swing by 11 days or more.
So if your cycle is 26 days one month and 31 the next, that’s well within the normal range. Consistently long cycles over 40 days, or cycles that are highly irregular over time, have been linked to higher risks of conditions like type 2 diabetes and heart disease. The American College of Obstetricians and Gynecologists considers the menstrual cycle a vital sign, on par with blood pressure and heart rate as an indicator of overall health.
How to Track Where You Are
You don’t need an app or a kit to get a general sense of your current phase, though both can help. The simplest method is paying attention to cervical mucus. After your period, discharge is dry or tacky. It becomes sticky and slightly damp around days 4 to 6, creamy and cloudy around days 7 to 9, then wet and stretchy around days 10 to 14 as you approach ovulation. After ovulation, it dries up again and stays that way until your next period.
Basal body temperature tracking adds another layer. You’ll need a thermometer sensitive to small changes, and you have to measure at the same time each morning before getting out of bed. A sustained rise of at least 0.3°C (about half a degree Fahrenheit) for three consecutive days confirms that ovulation has passed. Combining mucus observations with temperature tracking gives you a reasonably clear picture of your cycle without any special equipment.