Step by Step
1
Follicular phase (days 1-13)
FSH rises, recruiting primordial follicles until a dominant follicle emerges. As the dominant follicle grows, estrogen rises, causing the endometrium to proliferate. When estrogen reaches its peak, it triggers positive feedback, causing an LH surge.
2
Ovulation (day 14)
The LH surge causes the dominant follicle to rupture, releasing a secondary oocyte — which will only complete Meiosis II if it's actually fertilized.
3
Luteal phase (days 15-28)
The ruptured, now-empty follicle transforms into the corpus luteum, which produces progesterone (along with continued estrogen) — progesterone dominates this phase, causing the endometrium to become secretory, preparing for potential implantation.
4
Two possible outcomes
Without fertilization, the corpus luteum degenerates, progesterone and estrogen fall, and the endometrium is shed — menstruation. With fertilization, the developing trophoblast secretes hCG, which maintains the corpus luteum, allowing progesterone production to continue and preventing menstruation.
Applied Walkthrough
1
During the follicular phase, rising FSH recruits a group of primordial follicles, one of which becomes dominant — as it grows, it produces increasing amounts of estrogen, causing the endometrium to proliferate.
2
When estrogen peaks around day 13-14, it triggers an LH surge via positive feedback — causing the dominant follicle to rupture and release a secondary oocyte at ovulation.
3
The now-empty follicle transforms into the corpus luteum, shifting hormone dominance to progesterone during the luteal phase — preparing the endometrium to be receptive to implantation, should fertilization have occurred.
4
If fertilization did occur, the developing trophoblast secretes hCG, rescuing the corpus luteum and maintaining progesterone production; if not, the corpus luteum degenerates on its own, progesterone and estrogen fall, and the endometrium is shed as menstruation — restarting the entire cycle.
Exam Application
Exams test whether you can correctly sequence the three phases of the menstrual cycle (follicular, ovulation, luteal), identify which hormone dominates each phase, and explain what determines whether the cycle ends in menstruation or continues into pregnancy.
⚠ Common Trap
The most common trap is confusing which hormone dominates the follicular versus luteal phase — estrogen dominates the follicular phase (as the follicle develops), while progesterone dominates the luteal phase (from the corpus luteum) — mixing these up leads to incorrect predictions about endometrial changes.
✓ Quick Self-Check
1. What hormone rises during the follicular phase, and what effect does it have on the endometrium?
Estrogen; it causes the endometrium to proliferate.
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2. What triggers ovulation?
An LH surge, caused by positive feedback from peak estrogen levels.
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3. What structure forms after ovulation, and what hormone does it primarily produce?
The corpus luteum, primarily producing progesterone.
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4. What happens to the corpus luteum if fertilization doesn't occur?
It degenerates, causing progesterone and estrogen to fall and the endometrium to be shed (menstruation).
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5. What maintains the corpus luteum if fertilization does occur?
hCG, secreted by the developing trophoblast.
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