Menstrual Cycle

Aka: Menstrual Cycle, Follicular Phase, Proliferative Phase, Ovulatory Phase, Fertile Period, Luteal Phase

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II. Images

  1. menstrualCycle.png

III. Physiology: Background

  1. Menarche onset in girls (age <11 to 15 years)
    1. Menses onset after FSH and LH reach sufficient levels
  2. Oral Contraceptives (OCP) have a paradoxical effect on the Menstrual Cycle, Ovulation and fertility
    1. During a normal cycle, FSH triggers a dominant follicle to increase Estrogen, LH surge and Progesterone
    2. However chronically high Estrogen and Progesterone (e.g. OCP) inihibit Ovulation and fertilization
      1. Persistent Estrogen inhibits FSH and LH (resulting in no LH surge or Ovulation)
      2. Persistent Progesterone thickens Cervical Mucus and atrophies the uterine lining

IV. Physiology: Follicular Phase (Proliferative Phase in Early Cycle, Days 1 to 13 of cycle)

  1. Follicle Stimulating Hormone (FSH) stimulates ovary
    1. Group of ovarian follicles enlarge, of which one will be dominant, while the other follicles degenerate
    2. Dominant ovarian follicle emerges days 5-7 of cycle, increasing in size to 2 cm
      1. Vascularity doubles for dominant follicle
      2. Twin Gestation prone women generate 2 dominant follicles
      3. Dominant ovarian follicle produces Estrogen
      4. Dominant ovarian follicle also contains the ovum which will later be released during Ovulation
  2. Endometrium proliferative of glands and stroma
  3. Mean Duration: 10.8 days
  4. Hormones low
    1. Follicle Stimulating Hormone (FSH) low but rising
    2. Luteinizing Hormone (LH) low but rising
    3. Progesterone remains low

V. Physiology: Ovulatory Phase (Mid-Cycle)

  1. Hormone Surge
    1. Follicle Stimulating Hormone (FSH) with small surge
      1. Dominant ovarian follicle maturation is dependent on FSH stimulation
      2. Dominant ovarian follicle increases Estrogen release as it matures
    2. Estrogen rapidly peaks Days 10-15, then decreases
      1. Estrogen increases with dominant ovarian follicle maturation
      2. Rising Estrogen triggers a significant increase in LH (and to a lesser extent FSH)
        1. GnRH is released from Hypothalamus in pulses, in response to rising Estrogen
        2. GnRH stimulates pituitary LH and FSH release
        3. Contrast with chronically elevated Estrogen which suppresses LH and FSH release
        4. LH and FSH may be artificially stimulated in Female Infertility (e.g. Clomiphene)
    3. Luteinizing Hormone (LH) large surge
      1. Stimulates Ovulation (2 days after start of surge)
        1. Typically on day 14 of a 28 day cycle
        2. Occurs 14 days before Menstruation regardless of cycle length
      2. Stimulates corpus luteum
        1. Start of Progesterone increase, with endometrial secretory gland development
  2. Mid-cycle Symptoms Include
    1. Spotting
    2. Increased vaginal secretions
    3. Increased libido
    4. Nausea
    5. Abdominal Pain (Mittelschmerz)
  3. Identification of Ovulation Timing
    1. See Ovulation
    2. See Fertility Tracking
    3. See Basal Body Temperature
  4. Fertilization (pregnancy)
    1. Ovum (egg) fertilized typically within one day of Ovulation
    2. Sperm (viable for 1 to 3 days after intercourse) typically fertilizes the egg in the fallopian tube
    3. Fertilized egg exits the fallopian tube into the Uterus within 3-4 days after fertilization
      1. Ectopic Pregnancy may occur if the fertilized egg fails to reach the Uterus
    4. Fertilized egg implants within the endometrium 3-5 days after reaching the Uterus
      1. Implantation occurs 8 to 10 days after Ovulation
    5. Human Chorionic Gonadotropin (HCG) production starts on endometrial implantation
      1. See Pregnancy Test (Human Chorionic Gonadotropin)
      2. HCG (via an LH-like effect) maintains the corpus luteum
      3. Corpus luteum continues to produce Estrogen and Progesterone
      4. Estrogen and Progesterone maintain the endometrial lining and prevent sloughing
    6. Placenta develops from a combination of maternal and Embryonic tissue
      1. Also produces Estrogen and Progesterone

VI. Physiology: Luteal Phase (Secretory phase in Late Cycle)

  1. Endometrium with secretory gland development (stimulated by Progesterone)
    1. Prepares uterine endometrium for implantation (should an ovum be fertilized)
  2. Mean Duration: 13.3 days
  3. Hormonal changes: Ovum fertilization does not occur
    1. Estrogen continues to be high days 16-24
    2. Progesterone surges and remains until days 16-24
      1. Progesterone depletes Estrogen receptors
      2. Progesterone level falls to 0 at end of cycle
      3. Bleeding from Progesterone withdrawal occurs
    3. Increased Estrogen and Progesterone (as well as Inhibin) decrease FSH and LH via negative feedback
      1. Luteinizing Hormone (LH) returns to normal
      2. Follicle Stimulating Hormone (FSH) returns to normal
      3. Corpus luteum degenerates

VII. Physiology: Menstrual Phase (Bleeding)

  1. Corpus luteum degenerates
    1. Estrogen and Progesterone withdrawal
    2. Sloughing of endometrium with Menstrual Bleeding
      1. Fibrinolysin secreted by endometrium prevents menstrual blood from clotting
  2. LH and FSH start to increase again
    1. LH and FSH are no longer suppressed by Estrogen and Progesterone (via negative feedback)
    2. Starts a new Menstrual Cycle
    3. New group of ovarian follicles begins to develop
  3. Menopause
    1. Primordial follicles decrease over time, from Menarche, and eventually resulting in Menopause
    2. Estrogen and Progesterone production decreases as Menopause approaches
  4. Normal Menstrual Bleeding per cycle
    1. See Menses
    2. Duration: 2-8 days (mean: 4 days)
    3. Blood Loss: 20-80 ml (mean 35 ml), 13 mg iron loss
    4. Intervals: 21-35 days (mean 28 days)

VIII. References

  1. Goldberg (2014) Clinical Physiology, Medmasters, Miami, p. 143-5
  2. Guyton and Hall (2006) Medical Physiology, Elsevier Saunders, p. 1012-5

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