The menstrual cycle

Cycle axis and ovulation

• Pulsatile GnRH drives FSH and LH; FSH supports granulosa cells and follicle growth, while LH supports theca androgen production and ovulation.
• Sustained high estradiol flips feedback positive and triggers the LH surge; ovulation follows about 32-36 hours after surge onset.
• FSH surge is smaller than the LH surge; the LH surge resumes meiosis and luteinises the follicle.
• The follicular phase varies; the luteal phase is relatively constant at about 14 days.

Luteal and endometrial logic

• Corpus luteum makes progesterone, estradiol and inhibin A; hCG rescues it in early pregnancy.
• Estradiol produces proliferative endometrium; progesterone produces secretory endometrium.
• Subnuclear vacuoles are the classic early histological sign of progesterone action after ovulation.
• Menstruation is progesterone withdrawal with prostaglandin rise, spiral-arteriole constriction and functionalis shedding.

Timing and fertile window

• Estimate ovulation by counting back about 14 days from the next period.
• Fertile window spans the 5 days before ovulation and the day of ovulation.
• Mid-luteal progesterone means about 7 days before expected menses, not automatically day 21.
• Estrogenic cervical mucus is thin, stretchy and sperm-permeable; progesterone makes it thick.

AUB and pain

• AUB is described by frequency, regularity, duration and volume; exclude pregnancy first in reproductive-age patients.
• PALM structural causes are polyp, adenomyosis, leiomyoma, malignancy and hyperplasia.
• COEIN non-structural causes are coagulopathy, ovulatory dysfunction, endometrial, iatrogenic and not yet classified.
• Primary dysmenorrhoea is prostaglandin pain with a normal pelvis; secondary dysmenorrhoea means search for disease.