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Menstruation Explained: Physiology, Cycle Phases & Common Symptoms

Menstruation (menses, period) is the cyclic shedding of the endometrial lining of the uterus, accompanied by blood and cervical/vaginal secretions, exiting through the vagina. It marks the beginning of the menstrual cycle.

Physiology: The Hormonal Cycle (Average 28 Days)

  1. Menstrual Phase (Days 1–5): Progesterone/estrogen drop → spiral arteries constrict → endometrial ischemia → shedding. Average blood loss: 30–80 mL.
  2. Follicular Phase (Days 1–13): FSH ↑ → follicular recruitment → dominant follicle secretes estradiol → endometrial proliferation (rebuilding).
  3. Ovulation (~Day 14): LH surge → follicular rupture → oocyte release. Fertile window: 5 days before + day of ovulation.
  4. Luteal Phase (Days 15–28): Corpus luteum secretes progesterone (+ estradiol) → secretory endometrium (receptive for implantation). If no pregnancy → corpus luteum degenerates → hormone drop → menses.

Key Milestones

  • Menarche: First period. Average 12.4 years (US); range 9–15. Earlier with higher BMI, stress, endocrine disruptors.
  • Menopause: Final period (12 months amenorrhea). Average 51 years. Perimenopause: 4–8 years prior with irregular cycles.

Normal Variation

  • Cycle length: 21–35 days (adults); 21–45 days (adolescents).
  • Duration: 2–7 days.
  • Flow: Light to heavy (soaking ≥1 pad/tampon q1–2h = heavy).

Common Symptoms (Dysmenorrhea / PMS / PMDD)

  • Primary dysmenorrhea: Cramping (prostaglandin-mediated uterine contractions). NSAIDs, heat, exercise help.
  • Secondary dysmenorrhea: Caused by pathology (endometriosis, adenomyosis, fibroids). Requires evaluation.
  • PMS: Mood swings, bloating, breast tenderness, fatigue, acne in luteal phase. Resolves with menses.
  • PMDD: Severe affective/cognitive symptoms impairing function. Requires treatment (SSRIs, OCPs, GnRH analogs).

When to See a Clinician

  • No menarche by 15 (or 13 without secondary sexual characteristics).
  • Cycles <21 or >45 days (adults); >90 days amenorrhea.
  • Heavy menstrual bleeding (HMB): >80 mL or quality-of-life impact.
  • Severe pain unresponsive to NSAIDs.
  • Bleeding between periods, after sex, or postmenopause.

Menstrual Health = Vital Sign

ACOG recommends evaluating the menstrual cycle as a fifth vital sign. Tracking (apps, calendar) empowers patients and aids diagnosis.

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