The menstrual cycle is often taught as a monthly bleed and little else. In reality, it is a coordinated hormonal event that reshapes your uterus, your ovaries, your brain chemistry, and your metabolism roughly every 28 days, give or take. Learning its structure is not just useful for people trying to conceive or avoid conception. It is a baseline literacy for understanding your own physiology.
The American College of Obstetricians and Gynecologists (ACOG) frames the cycle as four overlapping phases: menstruation, the follicular phase, ovulation, and the luteal phase. The clock starts on the first day of bleeding.
Menstruation: days 1 to about 5
When a pregnancy has not occurred, estrogen and progesterone fall sharply. The uterine lining, built up during the previous cycle, sheds. Cramping is driven by prostaglandins, local signaling molecules that contract uterine muscle. Blood loss varies widely between individuals but averages roughly 30 to 80 mL across the whole period.
Heavy, prolonged, or newly painful bleeding is not something to normalize. It is worth investigating for causes ranging from fibroids to thyroid dysfunction to bleeding disorders.
Follicular phase: days 1 to about 13
The follicular phase overlaps with menstruation and continues afterward. The pituitary releases follicle-stimulating hormone (FSH), which recruits a cohort of ovarian follicles. Usually one becomes dominant. As it matures, it produces rising levels of estrogen, which thickens the uterine lining and shifts cervical mucus toward a clear, stretchy consistency friendly to sperm.
Many people report higher energy, sharper mood, and better sleep in this window. Some of that is hormonal, some is the absence of premenstrual symptoms. The follicular phase is also the most variable in length, which is why cycle length itself varies.
Ovulation: around day 14
A surge of luteinizing hormone (LH) triggers the dominant follicle to release its egg. The egg is viable for roughly 12 to 24 hours. Sperm, by contrast, can survive in the reproductive tract for up to five days, which is why the fertile window is wider than ovulation itself.
- Basal body temperature rises by roughly 0.3 to 0.5 degrees C after ovulation
- Some people feel a brief one-sided pelvic twinge called mittelschmerz
- Cervical mucus peaks in volume and elasticity just before ovulation
- LH surge is what at-home ovulation predictor kits detect
Ovulation does not always happen on day 14. In cycles that are shorter or longer than 28 days, the luteal phase stays relatively fixed at about 14 days, and it is the follicular phase that stretches or shortens.
Luteal phase: about day 15 to 28
After releasing the egg, the collapsed follicle becomes the corpus luteum, a temporary endocrine gland that secretes progesterone. Progesterone stabilizes the uterine lining in preparation for possible implantation. It also raises core body temperature, dampens some neurotransmitters, and can affect sleep, appetite, and mood in ways commonly grouped under premenstrual syndrome (PMS).
If implantation does not occur, the corpus luteum degenerates around day 24 to 26. Progesterone and estrogen fall, and the cycle begins again.
Severe premenstrual symptoms that reliably disrupt life for a week or more each month may meet criteria for premenstrual dysphoric disorder (PMDD), which has evidence-based treatments and is not something to endure silently.
What is normal, and what is not
A cycle length between 21 and 35 days, bleeding for two to seven days, and cycles that are reasonably predictable for you are all within typical range. What warrants a conversation with a clinician includes cycles consistently outside that window, bleeding heavy enough to cause anemia, severe pain that limits activity, bleeding between periods, or sudden changes after years of stability.
The bottom line
The menstrual cycle is a four-act sequence orchestrated by a small set of hormones. Understanding which phase you are in explains a great deal about how you feel, how your body responds to training and stress, and what your fertility looks like. It is also the frame doctors use when you describe symptoms, which makes cycle literacy a practical form of self-advocacy.