Chain 1: Why menstruation occurs

Corpus luteum lifespan ends (~14 days post-ovulation)
→ No hCG (no implantation)
→ Corpus luteum degenerates
→ Progesterone ↓ and Estrogen ↓
→ Vasoconstriction of spiral arteries in endometrium
→ Endometrial ischemia and necrosis
→ Endometrial lining shed = MENSTRUATION
→ Low $E_{2}$/$P_{4}$ removes negative feedback
→ FSH rises → new follicle recruited → next cycle

Chain 2: Why pregnancy stops menstruation

Trophoblast implants → secretes hCG
→ hCG acts on corpus luteum (like LH)
→ Corpus luteum maintained → progesterone high
→ Endometrium maintained (no shedding)
→ High $P_{4}$ + $E_{2}$ suppress FSH/LH (negative feedback)
→ No new follicle → no ovulation
→ No menstrual cycle → AMENORRHEA OF PREGNANCY

Chain 3: How LH surge leads to ovulation

Dominant follicle grows → granulosa cells secrete estrogen
→ Estrogen rises steadily → exceeds threshold
→ SWITCH: negative feedback → POSITIVE feedback on pituitary
→ Anterior pituitary surges LH (and smaller FSH surge)
→ LH acts on follicle → prostaglandins released
→ Follicular wall breaks down
→ Secondary oocyte + cumulus cells released = OVULATION
→ Remaining cells luteinize → corpus luteum
→ Corpus luteum secretes progesterone → luteal phase

Chain 4: Parturition positive feedback

Fetal maturation → fetal cortisol ↑
→ Placenta converts progesterone → estrogen
→ Estrogen dominance → myometrial oxytocin receptors ↑
→ Fetus presses on cervix → stretch receptors activated
→ Signals to hypothalamus → posterior pituitary releases OXYTOCIN
→ Oxytocin → myometrium contracts
→ Contractions → more cervical stretch → MORE OXYTOCIN
→ Positive feedback loop → escalating contractions → DELIVERY
→ Baby out → cervical stretch stops → loop breaks
→ Progesterone falls (placenta delivered) → prolactin acts → lactation