Menstrual Cycle Explained: Phases, Hormones, and What to Expect Each Month

The menstrual cycle is a coordinated endocrine‑uterine cycle preparing for potential pregnancy. Average length 28 days (21–35 days normal).

Phase 1: Menstrual Phase (Days 1–5)

Progesterone/estrogen drop → spiral artery vasoconstriction → functionalis layer of endometrium sheds → menstrual bleeding (30–80 mL).

Phase 2: Follicular Phase (Days 1–13)

• FSH ↑ → recruits ovarian follicles.
• Dominant follicle secretes estradiol → proliferative endometrium (glands, stroma thicken).
• Cervical mucus becomes clear, stretchy (spinnbarkeit).

Phase 3: Ovulation (Day ~14)

• Estradiol positive feedback → LH surge → follicular rupture → oocyte release.
• Basal body temperature rises 0.3–0.5°C post‑ovulation.

Phase 4: Luteal Phase (Days 15–28)

• Corpus luteum secretes progesterone (+ estradiol) → secretory endometrium (glycogen, spiral artery elongation).
• If no implantation: corpus luteum regresses → hormone drop → menses.
• If implantation: hCG rescues corpus luteum → progesterone maintains pregnancy.

Variability & When to Seek Care

• Cycle length, flow, symptoms vary widely.
• Consult clinician for: cycles <21 or >35 days, amenorrhea >3 months, heavy bleeding (>80 mL), severe dysmenorrhea, intermenstrual bleeding, suspected PCOS/thyroid disorder.