Menstrual Cycle and Hormonal Regulation

Chapter 3: The Menstrual Cycle and Hormonal Regulation

Overview of the Menstrual Cycle

  • The menstrual cycle involves complex interactions among the endometrium, ovaries, pituitary gland, and hypothalamus to prepare the body for fertilization.
  • Absence of fertilization leads to menstruation, which is the monthly shedding of the uterine lining.
  • Menstruation serves as both the beginning and end of the monthly cycle.
  • Menopause refers to the natural cessation of menstrual cycles.

Ovarian and Endometrial Cycles

  • The ovarian cycle (ovulation) and the endometrial cycle (menstruation) are distinct but interrelated, divided by the event of ovulation.
  • Ovulation is the release of the ovum from the ovarian follicle; the ovum then travels through the fallopian tube towards the uterus, where it may become fertilized by a sperm cell.

Hormones Involved

  • Primary hormones involved: GnRH, FSH, LH, estrogen, progesterone, and prostaglandins.
  • Gonadotropin-Releasing Hormone (GnRH):
    • Secreted from the hypothalamus; pulsates slowly during the follicular phase and increases during the luteal phase.
  • Follicle Stimulating Hormone (FSH):
    • Secreted by the anterior pituitary gland.
    • Responsible for the maturation of the ovarian follicle and the release of eggs.
  • Luteinizing Hormone (LH):
    • Produced and secreted by the anterior pituitary.
    • Essential for the final maturation of follicles and luteinization of the ruptured follicle.
    • Stimulates ovulation and the formation of the corpus luteum.
  • Estrogen:
    • A steroid hormone secreted by the ovaries.
    • Levels drop sharply post-ovulation, allowing progesterone to dominate.
    • Induces endometrial gland proliferation, increases uterine size and weight, enhances blood supply.
  • Progesterone:
    • Secreted by the corpus luteum post-ovulation.
    • Increases 5 to 7 days after ovulation.
    • Induces swelling and secretion in the endometrium to prepare for potential pregnancy.
    • Often referred to as the "hormone of pregnancy" due to its calming effect on the uterus.

Phases of the Ovarian Cycle

  1. Follicular Phase:

    • Begins on day 1 of the menstrual cycle, lasts until ovulation (approx. 10 to 14 days).
    • Follicles grow and mature to produce an ovum for fertilization.
    • Increase in estrogen from follicular cells stimulates proliferation of endometrium and myometrium.
    • If fertilization does not occur, the top layers of the endometrium shed during menstruation.
    • Hypothalamus prompts release of FSH from the pituitary, stimulating the growth of immature follicles.
    • Targeted follicle matures and prepares to release a mature oocyte through ovulation.
    • The end of the follicular phase is marked by an LH surge.

  2. Ovulation:

    • Defined as the release of an oocyte from the ovary into the fallopian tube for potential fertilization.
    • Triggered by a sharp surge in LH, characterized by the rupture of a mature follicle.
    • Occurs approximately on day 14 in a typical 28-day cycle.
    • Symptoms include:
      • Thin, clear, slippery cervical mucus to facilitate sperm travel.
      • Vaginal spotting, increased discharge, increased libido, slight rise in basal body temperature, and lower abdominal cramping (mittelschmerz).
    • Timing: Ovulation consistently occurs at least 14 days before menstruation begins.

  3. Luteal Phase:

    • Begins after ovulation and lasts from days 15 to 28 of a typical 28-day cycle.
    • Following ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone.
      • Progesterone induces endometrial glands to secrete glycogen and mucus, preparing for implantation.
    • Causes a rise in body temperature (by approx. 0.5°F to 1°F or 0.28°C to 0.56°C) that remains until 3 days before menstruation starts.
      • Sustained temperature increases may indicate early pregnancy.
    • In the absence of fertilization, the corpus luteum degenerates leading to reduced hormone levels, causing involution of the endometrium.
    • FSH and LH levels are lowest during the luteal phase.

Endometrial Cycle

  • Corresponds with the ovarian cycle and consists of three phases:
  1. Proliferative Phase:
    • Coincides with the follicular phase.
    • Following menses, estrogen stimulates growth of the endometry; glands enlarge, and blood vessels dilate.
    • This phase begins around day 5 and lasts until the time of ovulation.
  2. Secretory Phase:
    • Coincides with the luteal phase, from ovulation until about 3 days before the next menstrual period.
    • Progesterone influences thickened, vascular, and glandular endometrium that secretes glycogen and lipids in preparation for implantation.
    • If no fertilization occurs, hormone levels drop, leading to the beginning of menstruation.
  3. Ischemic Phase:
    • Initiates if fertilization does not occur, marked by sharp hormone level drops as the corpus luteum degenerates, resulting in endometrial ischemia and subsequent menstrual shedding.

Menstrual Phase

  • Characterized by the rupture of spiral arteries due to ischemia, leading to the shedding of the endometrial lining into the vagina.
  • Menstruation marks both the end of the current cycle and the start of a new one, with an average duration of bleeding up to 8 days.
  • Normal menstrual blood loss is considered less than 80 mL per cycle.
  • Menarche, the onset of menstruation, averages 12.4 years in the U.S., ranging from 10 to 16 years, influenced by genetics and other factors (Biro & Chan, 2023; Lacroix et al., 2023).

Puberty and Menstrual Irregularities

  • Events prior to menarche include:
    1. Thelarche: Development of breast buds.
    2. Adrenarche: Appearance of pubic/axillary hair and growth spurts.
    3. Menarche: Onset of the first menstrual cycle (occurs ~2 years after breast development).
  • Menstrual cycles can remain irregular for up to 2 years post-menarche; typical cycle regularity should be established thereafter.

Cultural Perspectives on Menstruation

  • Cultural attitudes toward menstruation vary significantly, affecting symptom expression and treatment-seeking behaviors.
  • Negative cultural beliefs can influence attitudes in young females regarding menstruation.
  • Education and improved access to menstrual products are essential for managing symptoms and promoting positive menstrual attitudes (Schmitt et al., 2022).

Perimenopause and Menopause

  • Perimenopause: Transitional period between onset of irregular cycles and the final menstrual period; signifies natural maturation of the reproductive system.
  • Menopause: The formal transition, defined as 1 year without a menstrual period, averaging around 51.4 years of age, with variations across individuals and populations.