First principle: three phases, two steroid regimes
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- Follicular phase — FSH‑driven recruitment and selection of a single dominant follicle (the one with the most FSH receptors/aromatase escapes the falling FSH as estrogen‑mediated negative feedback trims the cohort).
- Ovulation — the estrogen‑triggered LH surge.
- Luteal phase — progesterone‑dominant, and fixed at ~14 days unless rescued by hCG.
The clinically useful asymmetry
The luteal phase is constant (~14 days); the follicular phase varies. Therefore cycle‑length differences are almost entirely follicular‑phase differences, and ovulation dates ~14 days before the next menses — the logic behind all cycle timing (Concept 13).
Discriminators: which phase failed?
| Clue | Failing phase |
|---|---|
| No dominant follicle / chronic anovulation | follicular selection (e.g. PCOS) |
| Short luteal phase, low mid‑luteal progesterone | luteal insufficiency |
| No progesterone rise at all | no ovulation → no corpus luteum |
Pitfalls
- Assuming many antral follicles means good ovulation — in PCOS there are many follicles but failed selection and anovulation.
Pearls
- 🎯 Luteal phase = fixed 14 days. Memorise this; it underpins ovulation dating and mid‑luteal progesterone timing.
- 🩺 FSH also prevents atresia of the dominant follicle — which is why exogenous FSH rescues multiple follicles in stimulation (and risks OHSS).
Next → What flips estrogen from brake to trigger? (Concept 9).