First principle: an LH‑dependent, self‑limiting gland
The corpus luteum secretes progesterone (+ estrogen + inhibin A) and is maintained by LH. Without rescue it undergoes luteolysis at ~14 days → progesterone falls → menstruation.
Loading diagram…
The elegant hijack
hCG is an LH analogue. The implanting trophoblast secretes hCG to keep the corpus luteum producing progesterone until the luteal–placental shift (~7–10 weeks), after which the placenta is self‑sufficient. The same hCG is what a pregnancy test detects.
Clinical corollaries
- Corpus luteum cyst — a functional cyst; can cause pain or, if it bleeds, a haemorrhagic cyst (a torsion/rupture differential in early pregnancy).
- Luteal support — progesterone is supplemented in ART cycles because downregulation impairs endogenous luteal function.
Discriminators
- Falling β‑hCG in early pregnancy suggests failing trophoblast (non‑viable/ectopic); appropriately rising β‑hCG supports viability — the corpus luteum story made quantitative. 🚩
Pitfalls
- Forgetting that exogenous progesterone can delay menstruation and mask the diagnosis of pregnancy loss.
Pearls
- 🎯 hCG = LH mimic that rescues the corpus luteum; luteal phase is otherwise fixed at ~14 days.
- 🩺 The luteal–placental shift explains why very early pregnancies are corpus‑luteum‑dependent (relevant if the CL is removed surgically before ~10 weeks).
Next → What is the endometrium doing in parallel, and how is it dated? (Concept 11).