The simple idea (no memorising)
Follow the falling dominoes when there is no pregnancy:
The chemicals behind the vessel‑tightening and the cramping squeeze are called prostaglandins (especially one named PGF₂α) — remember that name; it returns in Concept 15 as the cause of period pain.
A picture for your mind
Progesterone was the caretaker keeping the guest room warm and lit. When the caretaker leaves, the heating switches off, the room is stripped, and everything is cleared out to start fresh. The period is that clear‑out — a sign the system is working, not failing.
Why it matters
Seeing a period as a progesterone‑withdrawal bleed is the key to understanding abnormal bleeding. If progesterone never showed up (because ovulation never happened), the lining just keeps thickening under estrogen with no orderly shedding — giving unpredictable, sometimes heavy bleeding. That's Concept 14.
Quick check ✅
- What fall in hormone triggers a period?
- Which chemicals cause the vessels to tighten and the womb to cramp?
Remember this 🌟
Where we're going next → How long should all this take, and when in the month is a woman fertile? Concept 13.