Overview: What is the menstrual cycle?
The menstrual cycle is a monthly series of hormonal and physical changes that prepares the uterus for potential pregnancy. According to ACOG, a normal cycle ranges from 21 to 35 days, with an average of 28 days — though only about 13% of people have exactly 28-day cycles. Day 1 is the first day of your period.
Phase 1 — Menstrual phase (days 1–5)
The menstrual phase begins when a pregnancy doesn't occur: falling progesterone and estrogen levels cause the uterine lining (endometrium) to shed. This is your period. The average period lasts 3–7 days and involves bleeding of about 30–80 ml.
Hormones: Both estrogen and progesterone are at their lowest. FSH begins to rise, signalling the start of the follicular phase.
Common experiences: Cramping (from prostaglandins that trigger uterine contractions), fatigue, lower back pain, mood changes. Physical activity can help reduce cramping via endorphin release.
Phase 2 — Follicular phase (days 1–13)
The follicular phase overlaps with the menstrual phase — it begins on day 1 and runs until ovulation. The pituitary gland releases FSH (follicle-stimulating hormone), which stimulates several follicles in the ovaries to develop. Each follicle contains an egg, but usually only one matures fully (the dominant follicle).
Hormones: Rising estrogen from the growing follicles causes the uterine lining to thicken (proliferative endometrium). As estrogen peaks, it triggers a surge of LH (luteinizing hormone).
Common experiences: Rising energy, improved mood, clearer skin. Estrogen has a mood-stabilising and libido-boosting effect for many people. Cervical mucus becomes more abundant and increasingly clear and stretchy — often described as "egg white" consistency close to ovulation.
Phase 3 — Ovulation (day 14 on a 28-day cycle)
Ovulation is the release of a mature egg from the dominant follicle, triggered by the LH surge (typically 24–36 hours after the surge peaks). The egg travels down the fallopian tube, where it is viable for 12–24 hours.
Hormones: The LH surge peaks just before ovulation. Estrogen peaks, then briefly dips after ovulation. A small amount of progesterone begins rising.
Common experiences: Mid-cycle spotting (in about 1 in 5 people), a slight rise in basal body temperature (BBT, typically 0.2–0.5°C), and a change in cervical position and mucus. Some people feel mittelschmerz — mild one-sided pelvic pain from the follicle rupturing.
The fertile window covers the 5 days before ovulation plus the ovulation day itself. See our fertile window guide for detail.
Phase 4 — Luteal phase (days 15–28)
After ovulation, the ruptured follicle transforms into the corpus luteum — a temporary glandular structure that secretes progesterone and some estrogen. If a fertilised egg implants, the corpus luteum persists (supported by hCG from the embryo) and continues producing progesterone to maintain the pregnancy. If no implantation occurs, the corpus luteum degenerates after approximately 10–16 days, causing progesterone and estrogen to fall, which triggers menstruation.
Hormones: Progesterone is dominant, causing the uterine lining to thicken and develop glands (secretory endometrium). Body temperature remains elevated until progesterone drops.
Common experiences: PMS symptoms (premenstrual syndrome) typically appear in the second half of the luteal phase, driven by falling estrogen and progesterone: bloating, breast tenderness, mood changes, food cravings, and fatigue. See our PMS guide for the full symptom list.
Quick reference — 28-day cycle: Menstrual (days 1–5) → Follicular (days 1–13) → Ovulation (day 14) → Luteal (days 15–28)
What controls cycle length?
The follicular phase is the most variable part of the cycle. When someone has a 35-day cycle, the extra 7 days are almost entirely in the follicular phase — ovulation just happens later. The luteal phase stays relatively constant at 12–16 days for most people.
Factors that can delay ovulation (and therefore extend the cycle): stress, illness, significant weight loss or gain, intense exercise, thyroid disorders, and polycystic ovary syndrome (PCOS). See our irregular periods guide for more.