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Menstrual Cycle &
Hormonal Regulation

Explore the four phases of the menstrual cycle, the hormonal feedback loops that regulate it, and how the hypothalamus, pituitary, and ovaries coordinate reproduction.

Anatomy & Physiology

What Is the Menstrual Cycle?

The menstrual cycle is a recurring series of physiological changes in the uterus and ovaries that makes pregnancy possible. It is driven by a precisely timed cascade of hormones originating from the hypothalamus, anterior pituitary gland, and the ovaries themselves.

The Key Hormones

GnRH
Released by the hypothalamus in pulses; triggers FSH and LH release from the anterior pituitary.
FSH
Follicle-stimulating hormone: stimulates ovarian follicle growth and estrogen production.
LH
Luteinizing hormone: triggers ovulation and supports the corpus luteum's progesterone secretion.
Estrogen & Progesterone
Ovarian steroids that regulate the uterine lining and feed back to the hypothalamus and pituitary.

The HPO Axis

Hypothalamus GnRH Anterior Pituitary FSH LH Ovary Ovary Estrogen / Progesterone (feedback) Inhibin (negative feedback) Uterus

The Four Phases of the Cycle

1
Menstrual Phase (Days 1-5): The endometrium sheds, causing menstrual bleeding. Hormone levels are at their lowest.
2
Follicular Phase (Days 1-13): FSH stimulates follicle growth. Rising estrogen rebuilds the uterine lining.
3
Ovulation (Day 14): A surge of LH triggers the release of a mature oocyte from the dominant follicle.
4
Luteal Phase (Days 15-28): The corpus luteum secretes progesterone, stabilizing the endometrium for possible implantation.

Hormone Levels Across the Cycle

Hormone Level 1 5 14 28 Day of Cycle Menses Follicular Luteal Ovulation Estrogen LH FSH Progesterone

Negative vs. Positive Feedback

Negative Feedback
Low-to-moderate estrogen and progesterone inhibit GnRH, FSH, and LH release. This keeps hormone levels stable during most of the cycle and prevents multiple follicles from maturing.
Positive Feedback
When estrogen crosses a critical threshold (around Day 12-13), it stimulates a massive LH surge from the pituitary. This positive feedback loop triggers ovulation.

Ovarian vs. Uterine Cycle

Ovarian Cycle Uterine Cycle
Phase 1 Follicular phase Menstrual + Proliferative
Key Event Follicle maturation Endometrium rebuilds
Phase 2 Luteal phase Secretory phase
Key Event Corpus luteum forms Endometrium thickens, glands secrete
Driven By FSH + LH Estrogen + Progesterone

The Follicular Phase in Detail

Primordial Primary Secondary Graafian (Mature) FSH stimulates growth Secretes estrogen

FSH recruits multiple follicles, but typically only one dominant follicle matures fully. As it grows, it secretes increasing amounts of estrogen, which thickens the endometrium and eventually triggers the LH surge.

The LH Surge and Ovulation

Ovary Oocyte LH Surge Corpus Luteum transforms into

The Luteal Phase

After ovulation, the corpus luteum secretes large amounts of progesterone and moderate estrogen. These hormones prepare the endometrium for possible implantation by:

1
Thickening the endometrium and increasing its vascularity (blood supply).
2
Stimulating uterine glands to secrete glycogen-rich fluid that nourishes a potential embryo.
3
Suppressing FSH and LH via negative feedback, preventing new follicles from developing.

What If Pregnancy Occurs?

Blastocyst implants Endometrium (maintained) hCG secreted Corpus Luteum (persists) Progesterone stays high No menstruation Cycle paused

How to Trace the Cycle

1
Identify the phase: Determine the day number and locate it within the menstrual, follicular, ovulatory, or luteal phase.
2
Identify the dominant hormone: FSH early, estrogen mid-follicular, LH at ovulation, progesterone in the luteal phase.
3
Determine the feedback type: Is the feedback negative (suppressing) or positive (amplifying)?
4
Link to the uterus: What is happening to the endometrium at this stage? Is it shedding, rebuilding, or secreting?
5
Predict what comes next: If the dominant hormone rises or falls, what events will that trigger?

Example: Day 7 of the Cycle

Scenario: A patient's blood work on Day 7 shows rising estrogen, moderate FSH, and low LH and progesterone. What is happening?

1
Phase: Day 7 falls in the follicular phase (Days 1-13).
2
Ovarian event: FSH is driving follicle growth. One dominant follicle is emerging and secreting estrogen.
3
Uterine event: The proliferative phase is underway: estrogen stimulates endometrial growth.
4
Feedback: Estrogen is still at moderate levels, so it exerts negative feedback on the pituitary.

Example: Day 21 of the Cycle

Scenario: Lab results on Day 21 show high progesterone, moderate estrogen, and low FSH and LH. Is this normal?

1
Phase: Day 21 is the mid-luteal phase (Days 15-28). This is the peak of corpus luteum activity.
2
Expected hormone profile: Progesterone is at its highest. Estrogen has a modest secondary peak. FSH and LH are suppressed by negative feedback.
3
Conclusion: This profile is completely normal. The endometrium is in its secretory phase, ready for implantation.

Common Mistakes

✕ Confusing Feedback Types
Estrogen does both negative and positive feedback. The switch depends on concentration and duration, not on the hormone itself.
✕ Mixing Up FSH and LH Roles
FSH drives follicle growth; LH triggers ovulation and supports the corpus luteum. They are not interchangeable.
✕ Forgetting Progesterone's Source
Progesterone rises only after ovulation, from the corpus luteum. It is not produced by the growing follicle in significant amounts.

Summary Table

Phase Days Key Hormone Key Event
Menstrual 1-5 All low Endometrium sheds
Follicular 1-13 FSH, rising estrogen Follicle matures
Ovulation ~14 LH surge Oocyte released
Luteal 15-28 Progesterone Endometrium maintained

Question 1 of 5

Which hormone is directly responsible for triggering ovulation?

Question 2 of 5

During the luteal phase, which structure is the primary source of progesterone?

Question 3 of 5

High levels of estrogen just before ovulation exert positive feedback on the pituitary. What does this mean?

Question 4 of 5

If the corpus luteum degenerates and no pregnancy occurs, what is the immediate consequence?

Question 5 of 5

A pregnancy test detects hCG in a patient's urine. What is the role of hCG in early pregnancy?