The Menstrual Cycle and Hormone Interaction

This lesson provides a deeper examination of the interactions between the four key hormones of the menstrual cycle, as required by the AQA GCSE Combined Science Trilogy specification (8464). Understanding these interactions — including feedback loops — is essential for achieving the highest grades.

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Recap: The Four Hormones

Hormone	Source	Primary Role
FSH	Pituitary gland	Stimulates egg maturation in the ovary; stimulates oestrogen production
Oestrogen	Ovaries	Thickens the uterus lining; inhibits FSH; stimulates LH release
LH	Pituitary gland	Triggers ovulation (~day 14)
Progesterone	Ovaries (corpus luteum)	Maintains the uterus lining; inhibits FSH and LH

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Detailed Hormone Interactions Throughout the Cycle

Days 1–5: Menstruation

• Progesterone levels have dropped (the corpus luteum has broken down).
• Without progesterone, the uterus lining cannot be maintained, so it breaks down and is shed (menstruation / period).
• Low levels of progesterone and oestrogen mean the pituitary gland is no longer inhibited.
• The pituitary gland begins to release FSH again.

Days 6–13: Follicular Phase

• FSH stimulates a follicle (containing an immature egg) to develop in one of the ovaries.
• As the follicle grows, it produces increasing amounts of oestrogen.
• Oestrogen causes the uterus lining to thicken and become rich in blood vessels, preparing for possible implantation.
• Rising oestrogen inhibits further FSH release from the pituitary gland (negative feedback) — this prevents more than one egg from maturing.
• As oestrogen levels continue to rise, they reach a threshold that triggers the pituitary gland to release a surge of LH (positive feedback).

Day 14: Ovulation

• The LH surge triggers ovulation — the release of the mature egg from the ovary.
• The egg travels into the oviduct (Fallopian tube), where it may be fertilised by a sperm cell.

Days 15–28: Luteal Phase

• After ovulation, the empty follicle in the ovary becomes the corpus luteum.
• The corpus luteum produces large amounts of progesterone (and some oestrogen).
• Progesterone maintains the thick, blood-rich uterus lining.
• Progesterone inhibits both FSH and LH from the pituitary gland — this prevents further ovulation.
• If the egg is not fertilised, the corpus luteum breaks down after about 10 days.
• Progesterone levels fall, the lining can no longer be maintained, and the cycle restarts at day 1.

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Hormone Level Changes — Timeline

flowchart LR
    subgraph "Days 1-5"
        A["Low oestrogen\nLow progesterone\n→ Lining sheds\n→ FSH begins to rise"]
    end
    subgraph "Days 6-13"
        B["FSH stimulates follicle\n→ Oestrogen rises\n→ Lining thickens\n→ FSH inhibited\n→ LH stimulated"]
    end
    subgraph "Day 14"
        C["LH surge\n→ OVULATION\n→ Egg released"]
    end
    subgraph "Days 15-28"
        D["Corpus luteum forms\n→ Progesterone rises\n→ Lining maintained\n→ FSH and LH inhibited"]
    end
    A --> B --> C --> D
    D -->|"No fertilisation:\nprogesterone drops"| A

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Positive and Negative Feedback in the Menstrual Cycle

The menstrual cycle uses both negative and positive feedback:

Feedback Type	Example in the Menstrual Cycle
Negative feedback	Rising oestrogen inhibits FSH (prevents multiple eggs maturing)
Negative feedback	Progesterone inhibits FSH and LH (prevents ovulation in the second half of the cycle)
Positive feedback	High oestrogen stimulates LH release (triggers the LH surge that causes ovulation)

Exam Tip: The switch from negative to positive feedback with oestrogen is a subtle but important point. At low levels, oestrogen inhibits FSH (negative feedback). At high levels, oestrogen stimulates the LH surge (positive feedback). This is a common higher-tier question.

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What Happens If Fertilisation Occurs?

If the egg is fertilised and implants in the uterus lining:

1. The developing embryo (and later the placenta) produces hCG (human chorionic gonadotrophin).
2. hCG keeps the corpus luteum alive, so it continues to produce progesterone.
3. Progesterone maintains the uterus lining throughout pregnancy.
4. High progesterone inhibits FSH and LH — so no new eggs mature and no menstruation occurs.

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Worked Example

Explain why a woman taking a progesterone-only contraceptive pill does not ovulate.

Model answer:

1. The pill provides a continuous supply of progesterone.
2. Progesterone inhibits the release of FSH from the pituitary gland.
3. Without FSH, no follicle develops and no egg matures in the ovary.
4. Progesterone also inhibits LH release.
5. Without the LH surge, ovulation cannot occur.

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Hormone Interaction Summary Table

Hormone	Stimulates	Inhibits
FSH	Egg maturation; oestrogen production	—
Oestrogen	LH release (at high levels); uterus lining thickening	FSH release (at low/moderate levels)
LH	Ovulation	—
Progesterone	Maintenance of uterus lining	FSH and LH release

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Common Mistakes

• Saying oestrogen only inhibits FSH — at high levels, oestrogen also stimulates LH (positive feedback). This distinction is important.
• Forgetting the corpus luteum — after ovulation, the empty follicle becomes the corpus luteum, which produces progesterone. Many students forget this step.
• Confusing the order of events — always follow the sequence: FSH → egg maturation → oestrogen → LH surge → ovulation → corpus luteum → progesterone.
• Not linking contraception to the cycle — the pill works because it artificially maintains hormone levels that inhibit FSH and/or LH.

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Summary

• The menstrual cycle is controlled by the interaction of FSH, oestrogen, LH, and progesterone.
• Negative feedback prevents multiple eggs from maturing and prevents ovulation in the second half of the cycle.
• Positive feedback (high oestrogen triggering the LH surge) causes ovulation.
• If fertilisation does not occur, the corpus luteum breaks down, progesterone falls, and the cycle restarts.
• Understanding these interactions is essential for exam success in AQA GCSE Combined Science Trilogy (8464).

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Extended Examples and Exam Technique

Extended Worked Example — Interpreting a Hormone Graph

An AQA data question may give you a graph showing the four hormones across a 28-day cycle. Typical features: