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 hormone levels that prepare the body for a potential pregnancy each month. Understanding what happens in each phase helps you recognize patterns in your energy, mood, fertility, and physical symptoms throughout the month.
Not everyone runs on a 28-day clock. Adult cycles typically fall between 21 and 34 days, and for adolescents in the first few years after their period starts, cycles anywhere from 21 to 45 days are considered normal. When cycles vary in length from month to month, the follicular phase is almost always the reason. It’s the most variable phase both between different people and within the same person over time. The luteal phase, by contrast, stays relatively consistent.
The Menstrual Phase (Days 1 to 5)
Day one of your period is day one of your cycle. This phase begins because a pregnancy didn’t occur in the previous cycle, so the hormones estrogen and progesterone drop sharply. Without those hormones sustaining it, the blood supply to the uterine lining cuts off and the tissue breaks down. The lining, which had built up to about 12 to 15 millimeters thick, sheds entirely over the course of a few days.
The uterus contracts to help push the shed tissue out along with blood and fluid. This is what causes cramping. Menstrual bleeding typically lasts 3 to 5 days, though anywhere from 2 to 7 days falls within the normal range. Hormone levels are at their lowest point during this phase, which is why many people feel fatigued or low-energy in the first days of their period.
The Follicular Phase (Days 1 to 13)
The follicular phase actually overlaps with menstruation. It starts on the same day your period begins and lasts until ovulation. During this stretch, your brain’s pituitary gland releases a hormone called FSH (follicle-stimulating hormone), which signals the ovaries to start developing follicles. Each follicle is a small fluid-filled sac containing an immature egg. Several follicles begin developing, but typically only one becomes dominant and continues maturing.
As the dominant follicle grows, it releases increasing amounts of estrogen. This rising estrogen does two important things: it triggers the uterine lining to start thickening again (rebuilding what was just shed during your period), and it eventually signals the brain that an egg is nearly ready for release. Energy and mood often improve during this phase as estrogen climbs. You may also notice changes in cervical mucus. Early in the follicular phase, mucus tends to be thick, white, and sticky, or barely present at all.
Ovulation (Around Day 14)
Ovulation is the shortest phase, lasting roughly 24 hours. It’s triggered when estrogen levels peak high enough to cause a surge of luteinizing hormone (LH). This LH surge happens about 36 to 40 hours before the egg is actually released from the ovary. Home ovulation test kits detect LH in urine, and once that surge is detected, ovulation typically follows within 12 to 24 hours.
The released 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, though sperm can survive inside the body for several days, meaning the days leading up to ovulation are also fertile.
Your body gives a few physical signals around ovulation. Cervical mucus becomes clear, slippery, and stretchy, often compared to raw egg whites. This thinner consistency makes it easier for sperm to travel through the cervix. Basal body temperature (your resting temperature first thing in the morning) also rises slightly after ovulation, typically by less than half a degree Fahrenheit (about 0.3°C). That temperature shift confirms ovulation has already happened, so it’s useful for tracking patterns over several months rather than predicting ovulation in the moment.
The Luteal Phase (Days 15 to 28)
After the egg is released, the empty follicle left behind on the ovary transforms into a structure called the corpus luteum. This structure produces progesterone, along with some estrogen, and progesterone becomes the dominant hormone for the rest of the cycle. Progesterone’s job is to prepare the uterine lining for a potential pregnancy. It thickens the lining further, creating an environment where a fertilized egg could implant and begin growing. It also thickens cervical mucus into a paste-like consistency, which acts as a barrier against bacteria entering the uterus.
If the egg isn’t fertilized, the corpus luteum breaks down after about 10 to 14 days. As it degenerates, progesterone and estrogen levels plummet, and the cycle resets. That hormone drop is what triggers the uterine lining to shed again, starting a new period and a new cycle.
The second half of the luteal phase, when progesterone starts falling, is when premenstrual symptoms tend to appear. Bloating, breast tenderness, mood changes, food cravings, and fatigue are all linked to this hormonal shift. If a fertilized egg does implant, the corpus luteum keeps producing progesterone to sustain the early pregnancy, which is why a missed period is one of the first signs of conception.
Tracking Your Cycle
Two of the simplest indicators you can monitor at home are cervical mucus and basal body temperature. Mucus follows a predictable pattern: thick or dry after your period, gradually becoming wetter and more slippery as ovulation approaches, then returning to thick and dry in the luteal phase. Temperature tracking requires taking your temperature at the same time every morning before getting out of bed. You’re looking for a small sustained rise that confirms ovulation occurred.
Neither method alone is perfectly reliable for predicting fertility, but together they give you a useful picture of where you are in your cycle. Tracking over several months also helps you spot irregularities. If your cycles consistently fall outside the 21-to-34-day range, or if you notice you’re not seeing any temperature shift (which can suggest cycles without ovulation), that’s worth discussing with a healthcare provider.