11.3 Menstrual Cycle and Hormonal Regulation

The menstrual cycle is a complex dance of hormones orchestrating female reproductive function. It involves intricate interplay between the brain, pituitary gland, and ovaries, regulating follicle development, ovulation, and uterine preparation for potential pregnancy.

Understanding this cycle is crucial for grasping female reproductive physiology. It explains fertility patterns, menstruation, and hormonal fluctuations that impact various bodily systems. This knowledge is essential for family planning, diagnosing reproductive issues, and comprehending overall female health.

Phases of the Menstrual Cycle and Hormonal Changes

Cycle Overview and Follicular Phase

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• Menstrual cycle occurs over a 28-day period divided into three main phases
  • Follicular phase
  • Ovulation
  • Luteal phase
• Follicular phase characterized by ovarian follicle development
  • Stimulated by follicle-stimulating hormone (FSH)
  • Increasing estrogen levels produced by developing follicles
• Gonadotropin-releasing hormone (GnRH) from hypothalamus regulates FSH and LH release
  • GnRH secreted in pulsatile manner
  • Acts on anterior pituitary gland

Ovulation and Luteal Phase

• Ovulation occurs midcycle triggered by luteinizing hormone (LH) surge
  • Mature ovum released from dominant follicle
  • Usually happens around day 14 of a 28-day cycle
• Luteal phase marked by corpus luteum formation
  • Remnant of ruptured follicle
  • Secretes progesterone and estrogen
  • Prepares uterus for potential implantation
• Estrogen levels peak twice during cycle
  • First peak before ovulation
  • Second peak during luteal phase
• Progesterone levels rise significantly only during luteal phase

Cycle Completion and Hormonal Fluctuations

• If fertilization does not occur, corpus luteum degenerates
  • Leads to drop in hormone levels
  • Triggers menstruation, marking start of new cycle
• FSH levels begin to rise as negative feedback decreases
  • Initiates follicle development for next cycle
• Inhibin produced by developing follicles
  • Acts as negative feedback regulator of FSH
  • Helps in selection of dominant follicle

Roles of Key Hormones in Menstrual Cycle Regulation

Gonadotropins: FSH and LH

• Follicle-stimulating hormone (FSH) stimulates ovarian follicle growth
  • Promotes estrogen production by granulosa cells
  • Levels highest during early follicular phase
• Luteinizing hormone (LH) triggers ovulation
  • Stimulates corpus luteum formation
  • Promotes progesterone production by luteal cells
  • Surges dramatically just before ovulation (around day 14)

Ovarian Hormones: Estrogen and Progesterone

• Estrogen primarily produced by developing follicles
  • Promotes endometrial proliferation
  • Changes cervical mucus to facilitate sperm transport
  • Peaks twice: before ovulation and during luteal phase
• Progesterone mainly secreted by corpus luteum
  • Prepares endometrium for potential implantation
  • Maintains pregnancy if fertilization occurs
  • Rises significantly only during luteal phase

Hormone Interactions and Feedback Systems

• Complex feedback system between hypothalamus, pituitary, and ovaries
  • Estrogen and progesterone modulate GnRH, FSH, and LH release
  • Ratio of estrogen to progesterone influences physiological changes
    • Affects body temperature (rises after ovulation)
    • Changes cervical mucus consistency (becomes more elastic near ovulation)
• Inhibin acts as negative feedback regulator of FSH
  • Produced by granulosa cells in developing follicles
  • Helps select dominant follicle by suppressing FSH

Endometrial Changes During Menstrual Cycle

Proliferative Phase

• Occurs during follicular phase of ovarian cycle
• Rising estrogen levels stimulate endometrial growth
  • Endometrium thickens
  • Glandular development increases
  • Vascularization enhances
• Endometrial thickness increases from 2-4 mm to 10-12 mm

Secretory Phase

• Initiated by progesterone after ovulation
• Characterized by increased glandular secretion
  • Glands become more coiled and secretory
  • Glycogen and lipid accumulation in glandular cells
• Further vascularization prepares for potential implantation
  • Spiral arteries develop in endometrium
  • Crucial for providing blood supply to potential embryo

Menstruation and Regeneration

• If fertilization does not occur, progesterone and estrogen levels drop
  • Triggers breakdown and shedding of endometrium (menstruation)
  • Prostaglandins induce uterine contractions during menstruation
• Basal layer of endometrium remains intact
  • Serves as source for regeneration in subsequent cycle
• Menstrual flow typically lasts 3-7 days
  • Average blood loss 30-80 mL

Hypothalamic-Pituitary-Ovarian Axis Regulation

Positive and Negative Feedback Mechanisms

• Hypothalamic-pituitary-ovarian axis operates through feedback loops
• GnRH from hypothalamus stimulates FSH and LH release from anterior pituitary
• Early follicular phase: low estrogen exerts negative feedback
  • Suppresses GnRH, FSH, and LH secretion
• Late follicular phase: rising estrogen switches to positive feedback
  • Stimulates LH surge necessary for ovulation
  • Known as estrogen-induced LH surge

Post-Ovulatory Regulation

• After ovulation, corpus luteum secretes progesterone and estrogen
  • Exert negative feedback on GnRH, FSH, and LH secretion
• Decline of corpus luteum function reduces progesterone and estrogen
  • Decreases negative feedback
  • Allows FSH levels to rise, initiating new cycle

Role of Inhibin in Cycle Regulation

• Inhibin produced by developing follicles and corpus luteum
  • Selectively suppresses FSH secretion
  • Contributes to follicle selection
  • Helps maintain appropriate FSH levels throughout cycle
• Two forms: Inhibin A and Inhibin B
  • Inhibin B predominates in follicular phase
  • Inhibin A increases in luteal phase

Physiological and Behavioral Changes During Menstrual Cycle

Temperature and Cervical Mucus Changes

• Basal body temperature increases slightly after ovulation
  • Due to thermogenic effect of progesterone
  • Provides marker for fertility tracking
  • Typically rises by 0.2-0.5°C (0.4-1.0°F)
• Cervical mucus changes throughout cycle
  • Becomes more abundant, clear, and elastic near ovulation
  • Facilitates sperm transport
  • Changes from sticky to creamy to egg white consistency

Physical Symptoms and Libido

• Breast tenderness and bloating may occur during luteal phase
  • Due to fluid retention caused by hormonal fluctuations
• Libido often increases around ovulation
  • Potentially influenced by peak in estrogen and testosterone levels
• Some women experience premenstrual syndrome (PMS) symptoms
  • Occur in late luteal phase
  • Include mood swings, irritability, and physical discomfort

Cognitive and Energy Fluctuations

• Cognitive function may be subtly affected throughout cycle
  • Some studies suggest enhanced verbal fluency during late follicular phase
• Energy levels and metabolism can fluctuate
  • Increased energy often reported near ovulation
  • Fatigue more common during premenstrual period
• Sleep patterns may change
  • Some women report improved sleep quality during follicular phase
  • Insomnia more common in luteal phase

Fertility Awareness and Family Planning

Fertility Window and Conception Timing

• Fertility awareness methods (FAMs) track physiological signs of fertility
  • Used to identify fertile window for conception or contraception
• Fertile window typically spans 6 days
  • Includes 5 days before ovulation and day of ovulation
  • Due to sperm viability (up to 5 days) and egg viability (24 hours)
• Ovulation usually occurs 14 days before the start of next menstrual period
  • Can vary significantly between women and cycles

Tracking Methods and Indicators

• Basal body temperature (BBT) charting indicates ovulation
  • Detects post-ovulatory temperature rise
  • Retrospective indicator, useful for confirming ovulation
• Cervical mucus observations help predict ovulation approach
  • Mucus becomes more abundant and elastic near ovulation
  • "Egg white" consistency most fertile
• Ovulation predictor kits detect LH surge preceding ovulation
  • Provide more precise timing of fertile window
  • Typically turn positive 24-36 hours before ovulation

Combining Methods for Increased Accuracy

• Calendar method estimates fertility based on cycle length
  • Less reliable due to cycle variability
  • Can be used as a starting point for other methods
• Sympto-thermal method combines multiple indicators
  • Increases accuracy of fertility prediction
  • Typically uses BBT, cervical mucus, and calendar calculations
• Effectiveness of FAMs varies
  • Perfect use: 95-99% effective
  • Typical use: 76-88% effective
  • Requires consistent tracking and interpretation of signs