Menstrual Cycle Hormones: How Progesterone Withdrawal Triggers Menstruation
The menstrual cycle is orchestrated by the hypothalamic‑pituitary‑ovarian axis.
- Follicular phase: FSH → follicular growth → estradiol ↑ → endometrial proliferation.
- Ovulation: LH surge → oocyte release.
- Luteal phase: Corpus luteum secretes progesterone (+ estradiol) → secretory endometrium.
- Progesterone withdrawal: If no implantation, corpus luteum regresses → progesterone ↓ → spiral artery vasoconstriction → endometrial ischemia → shedding (menstruation).
- New cycle: Falling steroids remove negative feedback → FSH rises → new follicular cohort.
Progesterone is the key hormone maintaining the secretory endometrium; its withdrawal is the proximate trigger for menses.
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