Understanding the Menstrual Cycle

The Menstrual Cycle

• The menstrual cycle refers to cyclical changes in the endometrium (lining of the uterus) occurring monthly.
• A key feature is menstruation, the monthly flow of blood.
• The cycle is divided into postmenstrual, proliferative, secretory, and menstrual phases.
• Another division is the follicular phase (estrogen-driven) and the luteal phase (progesterone-driven).
• Endometrial changes include increased thickness, uterine gland growth, changes in epithelial cells, and thickened stroma.
• Blood supply to superficial endometrial parts is cut off before menstruation, causing shedding and bleeding.
• The cycle is influenced by hormones like estrogen, progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
• The reproductive period is when a woman can bear children, marked by monthly menstruation (menses).
• Menarche is the onset of menstruation at around 12 years of age.
• Menopause is the cessation of menstruation around 45 years of age.

Phases of the Menstrual Cycle

• The cycle is typically divided into postmenstrual, proliferative, secretory (or premenstrual), and menstrual phases, based on endometrial changes.
• The follicular phase (first half) is characterized by actions of oestrogen from growing follicles, leading to endometrial changes.
• The luteal phase (second half) is controlled by progesterone from the corpus luteum, with significant endometrial changes.

Uterine Structure and Changes

• The uterus has three layers: perimetrium (outermost), myometrium (muscle), and endometrium (innermost).
• The endometrium is the layer undergoing cyclical changes during the menstrual cycle.
• The stroma (connective tissue) is found in the endometrium, and its structure varies in the different phases.

Hormonal Control of the Menstrual Cycle

• Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary gland.
• The anterior pituitary releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which affect the ovaries.
• FSH stimulates follicle growth and oestrogen production in the ovaries.
• LH triggers ovulation and corpus luteum formation, leading to progesterone production.
• Oestrogen and progesterone have significant effects on endometrial changes.
• The cycles are closely interconnected. LH surge leads to ovulation, and in response, the corpus luteum forms and secretes progesterone.

Clinical Correlation

• Understanding ovulation timing (e.g., using temperature charts) is pertinent for family planning.
• Contraceptive pills often contain synthetic hormones mimicking oestrogen and/or progesterone, preventing ovulation.