SN1 SHORT NOTE 5M ★ High-yield

Describe the mechanism of ovulation and enumerate its indicators.

Physiology · 1st Professional — Pre-Clinical

Definition

Ovulation = the rupture of the mature Graafian follicle with release of the secondary oocyte into the peritoneal cavity, occurring on about day 14 of a 28-day cycle.

Mechanism

Sustained high oestrogen → positive feedback → LH surge → Resumption of the first meiotic division; first polar body extruded → ↑ Prostaglandins and proteolytic enzymes (collagenase, plasmin) → Follicular wall weakens → stigma forms → rupture — 24–36 h after the surge begins

Indicators of Ovulation

Indicator	Change
Basal body temperature	↑ 0.3–0.5 °C and sustained — a biphasic chart. Due to progesterone
Cervical mucus	Becomes thin, watery, profuse; spinnbarkeit > 8 cm; ferning positive
Serum progesterone (day 21)	> 5 ng/mL confirms ovulation
Urinary LH kit	Detects the surge; ovulation follows in 24–36 h
Ultrasound	Follicle > 18 mm, then its collapse
Endometrial biopsy	Secretory changes
Mittelschmerz	Mid-cycle lower abdominal pain
Vaginal cytology	Cornified cells

Fertile Period

Ovum survives 12–24 hours; sperm survive 48–72 hours
Fertile window therefore spans about day 10 to day 17 in a regular 28-day cycle
Basis of the rhythm (calendar) method — unreliable because the follicular phase varies

Applied Aspects

Anovulation — a major cause of infertility; commonest in PCOS
Ovulation induction — clomiphene blocks hypothalamic oestrogen receptors → ↑ FSH and LH; letrozole; gonadotrophins
Ovarian hyperstimulation syndrome is the chief hazard of induction
Emergency contraception works largely by delaying or preventing ovulation
Mittelschmerz and mid-cycle spotting are harmless but must not be mistaken for pathology
Luteinised unruptured follicle syndrome — the follicle luteinises without releasing the oocyte; progesterone rises although no ovulation has occurred, so a day-21 progesterone alone can mislead

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