The Menstrual Cycle

This lesson covers the hormonal control of the menstrual cycle — a critical topic in Edexcel GCSE Biology (1BI0) Topic 7. You must understand the roles of FSH, LH, oestrogen, and progesterone, how they interact, and how the cycle repeats approximately every 28 days.

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Overview of the Menstrual Cycle

The menstrual cycle is a recurring process lasting approximately 28 days that prepares the female body for pregnancy. It involves the maturation and release of an egg, and the build-up and shedding of the uterus lining (endometrium).

The cycle is controlled by four key hormones working together in a carefully coordinated sequence.

The Four Key Hormones

Hormone	Produced By	Key Functions
FSH (follicle-stimulating hormone)	Pituitary gland	Causes an egg to mature inside a follicle in the ovary; stimulates the ovary to produce oestrogen
Oestrogen	Ovaries (developing follicle)	Causes the uterus lining to thicken and develop a blood supply; triggers a surge of LH; inhibits further FSH production
LH (luteinising hormone)	Pituitary gland	Triggers ovulation — the release of the mature egg from the ovary (around day 14)
Progesterone	Ovaries (corpus luteum)	Maintains the thick uterus lining after ovulation; inhibits FSH and LH so no new eggs mature during this phase

Exam Tip: Remember the mnemonic "FOLP" — FSH, Oestrogen, LH, Progesterone — this is roughly the order in which each hormone dominates during the cycle.

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Day-by-Day: What Happens During the Cycle

Days 1–5: Menstruation

• The uterus lining breaks down and is shed, along with a small amount of blood. This is menstruation (a period).
• Hormone levels of oestrogen, progesterone, LH, and FSH are all at their lowest at the start.
• The pituitary gland begins to release FSH because progesterone levels have dropped (progesterone was inhibiting FSH; now the inhibition is removed).

Days 5–13: The Follicular Phase

• FSH stimulates a follicle (containing an immature egg) to develop in the ovary.
• As the follicle grows, it produces increasing amounts of oestrogen.
• Oestrogen causes the uterus lining to thicken and develop a rich blood supply, preparing for possible implantation of a fertilised egg.
• Rising oestrogen levels inhibit FSH production (negative feedback), preventing additional follicles from developing.
• When oestrogen reaches a high enough level, it triggers a sudden surge in LH from the pituitary gland (this is an example of positive feedback).

Day 14: Ovulation

• The LH surge triggers ovulation — the release of the mature egg from the follicle in the ovary.
• The egg is released into the oviduct (fallopian tube), where it may be fertilised by a sperm cell.
• LH levels peak sharply and then decline.

Days 15–28: The Luteal Phase

• After ovulation, the empty follicle transforms into a structure called the corpus luteum.
• The corpus luteum produces large amounts of progesterone (and some oestrogen).
• Progesterone maintains the thick uterus lining, keeping it ready for implantation.
• Progesterone also inhibits FSH and LH, preventing further eggs from maturing or being released.

If the Egg Is NOT Fertilised

• The corpus luteum gradually breaks down after about 10 days.
• As it degrades, progesterone levels drop.
• Without progesterone to maintain it, the uterus lining breaks down and is shed — this is menstruation, and the cycle begins again at Day 1.
• The drop in progesterone also removes the inhibition on FSH, so the pituitary gland starts releasing FSH again — and the whole cycle repeats.

If the Egg IS Fertilised

• The developing embryo produces a hormone called hCG (human chorionic gonadotrophin).
• hCG keeps the corpus luteum alive, so it continues producing progesterone.
• The uterus lining is maintained, and menstruation does not occur.

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Hormone Interactions — Feedback Loops

The hormones of the menstrual cycle interact through both negative feedback and positive feedback:

graph TD
    A["Pituitary gland releases FSH"] -->|"stimulates"| B["Follicle develops in ovary"]
    B -->|"produces"| C["Oestrogen rises"]
    C -->|"INHIBITS - negative feedback"| A
    C -->|"at high levels - STIMULATES - positive feedback"| D["Pituitary releases LH surge"]
    D -->|"triggers"| E["Ovulation on day 14"]
    E -->|"follicle becomes"| F["Corpus luteum"]
    F -->|"produces"| G["Progesterone rises"]
    G -->|"INHIBITS FSH and LH"| A
    G -->|"maintains"| H["Uterus lining"]
    G -->|"corpus luteum breaks down"| I["Progesterone drops"]
    I -->|"lining sheds"| J["Menstruation - cycle restarts"]
    J --> A

    style A fill:#9b59b6,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#9b59b6,color:#fff
    style E fill:#f1c40f,color:#333
    style G fill:#3498db,color:#fff
    style J fill:#e67e22,color:#fff

Negative Feedback Examples

• Oestrogen inhibits FSH: As the follicle grows and produces more oestrogen, FSH levels fall. This prevents multiple eggs from developing.
• Progesterone inhibits FSH and LH: After ovulation, high progesterone levels prevent the pituitary from releasing FSH or LH, stopping new follicles from developing during the luteal phase.

Positive Feedback Example

• Oestrogen stimulates LH: When oestrogen reaches a sufficiently high concentration, it triggers a surge of LH from the pituitary gland. This is positive feedback because oestrogen causes more LH to be released (the response amplifies the stimulus), which then causes ovulation.

Exam Tip: Examiners love asking about the interaction between oestrogen and FSH (negative feedback) and oestrogen and LH (positive feedback at high concentrations). Make sure you can explain both clearly.

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Hormone Levels — Graph Description

If you were to plot the levels of all four hormones over a 28-day cycle, you would see:

Day Range	FSH	Oestrogen	LH	Progesterone
1–5	Begins to rise	Low	Low	Low (falling)
5–13	Falls as oestrogen rises	Rising steadily	Low (then surges near day 13)	Low
~14	Low	Peaks then falls	Sharp peak (triggers ovulation)	Begins to rise
15–25	Low	Moderate	Low	High (maintains lining)
25–28	Begins to rise again	Falls	Low	Falls (corpus luteum breaks down)

Exam Tip: You may be given a graph of hormone levels and asked to identify what is happening at each stage. Remember: the LH spike always corresponds to ovulation around day 14. A drop in progesterone at the end of the cycle causes menstruation.

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Summary of Hormone Actions

Hormone	Where Produced	Stimulates	Inhibits
FSH	Pituitary gland	Follicle development; oestrogen production	—
Oestrogen	Ovaries	Uterus lining thickening; LH surge (at high levels)	FSH
LH	Pituitary gland	Ovulation (release of the egg)	—
Progesterone	Corpus luteum	Maintains uterus lining	FSH and LH

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Summary