Hormones in Human Reproduction

This lesson covers the role of hormones in puberty and the menstrual cycle. Understanding how reproductive hormones interact is a key part of the AQA GCSE Biology specification. This links closely to the next lesson on contraception and fertility treatments.

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Puberty and Sex Hormones

Puberty is the stage of development when the body becomes sexually mature and capable of reproduction. It is triggered by hormones released from the pituitary gland, which stimulate the reproductive organs to produce sex hormones.

Hormones in Male Puberty

The pituitary gland releases hormones that stimulate the testes to produce testosterone.

Change	Description
Growth of testes and penis	Reproductive organs enlarge
Pubic and body hair growth	Hair grows in the armpits, face, chest, and pubic region
Voice deepens	Larynx (voice box) grows, causing the voice to "break"
Muscle development	Increased muscle mass and strength
Growth spurt	Rapid increase in height
Sperm production begins	Testes start producing sperm cells
Body odour changes	Increased sweat gland activity

Hormones in Female Puberty

The pituitary gland releases FSH (follicle-stimulating hormone), which stimulates the ovaries to produce oestrogen.

Change	Description
Breast development	Mammary glands develop
Pubic and body hair growth	Hair grows in the armpits and pubic region
Hips widen	Pelvis broadens in preparation for childbearing
Growth spurt	Rapid increase in height
Menstruation begins	The menstrual cycle starts (menarche)
Egg maturation	Ovaries begin to mature and release eggs
Body fat redistribution	Fat is deposited around hips, thighs, and breasts

Exam Tip: The key hormone for male puberty is testosterone (from the testes). The key hormone for female puberty is oestrogen (from the ovaries). Both are triggered by hormones from the pituitary gland.

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The Menstrual Cycle

The menstrual cycle is a recurring process, approximately 28 days long, that prepares the female body for pregnancy. It involves the interaction of four key hormones.

The Four Key Hormones

Hormone	Produced By	Main Role
FSH (follicle-stimulating hormone)	Pituitary gland	Causes an egg to mature in a follicle in the ovary; stimulates the ovaries to produce oestrogen
Oestrogen	Ovaries (from the developing follicle)	Causes the uterus lining to thicken and grow; stimulates LH release; inhibits FSH production
LH (luteinising hormone)	Pituitary gland	Triggers ovulation — the release of the mature egg from the ovary (around day 14)
Progesterone	Ovaries (from the corpus luteum — the empty follicle after ovulation)	Maintains the thick uterus lining; inhibits FSH and LH production

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Stages of the Menstrual Cycle

Day 1-5: Menstruation

• The uterus lining breaks down and is shed through the vagina (menstruation or a "period").
• FSH levels begin to rise.

Day 5-14: Follicular Phase (Repair and Build-up)

• FSH from the pituitary gland stimulates an egg to mature inside a follicle in the ovary.
• The developing follicle produces oestrogen.
• Oestrogen causes the uterus lining to thicken with blood vessels and glandular tissue, preparing for a possible pregnancy.
• Rising oestrogen levels inhibit (reduce) FSH production (negative feedback).
• When oestrogen reaches a high level, it stimulates a surge of LH from the pituitary gland (positive feedback).

Day 14: Ovulation

• The surge of LH triggers ovulation — the mature egg is released from the follicle in the ovary into the fallopian tube (oviduct).
• This is the most fertile time in the cycle.

Day 14-28: Luteal Phase (Maintenance)

• The empty follicle becomes the corpus luteum, which produces progesterone.
• Progesterone maintains the thick uterus lining, keeping it ready for implantation of a fertilised egg.
• Progesterone inhibits FSH and LH production, preventing another egg from maturing.
• If the egg is not fertilised, the corpus luteum breaks down, progesterone levels fall, and the uterus lining is no longer maintained.
• The cycle restarts with menstruation.

flowchart TD
    A[Day 1-5: Menstruation] --> B[Uterus lining sheds]
    B --> C[FSH rises]
    C --> D[Day 5-14: Follicular Phase]
    D --> E[FSH stimulates egg maturation in follicle]
    E --> F[Follicle produces oestrogen]
    F --> G[Oestrogen thickens uterus lining]
    F --> H[High oestrogen stimulates LH surge]
    H --> I[Day 14: Ovulation - LH triggers egg release]
    I --> J[Day 14-28: Luteal Phase]
    J --> K[Corpus luteum produces progesterone]
    K --> L[Progesterone maintains uterus lining]
    K --> M[Progesterone inhibits FSH and LH]
    L --> N{Egg fertilised?}
    N -->|No| O[Corpus luteum breaks down]
    O --> P[Progesterone falls]
    P --> A
    N -->|Yes| Q[Pregnancy continues]

Exam Tip: A very common exam question asks you to describe the roles of the four hormones in the menstrual cycle. Remember the sequence: FSH matures the egg, oestrogen thickens the lining and triggers LH, LH causes ovulation, progesterone maintains the lining. Learn the interactions (which hormones stimulate or inhibit others).

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Hormone Interactions — Feedback Loops

The menstrual cycle is controlled by complex interactions between hormones:

Hormone	Stimulates	Inhibits
FSH	Egg maturation; oestrogen production	(Inhibited by oestrogen and progesterone)
Oestrogen	Uterus lining thickening; LH surge (at high levels)	FSH (negative feedback)
LH	Ovulation	(Inhibited by progesterone)
Progesterone	Maintenance of uterus lining	FSH and LH (negative feedback)

Key feedback relationships:

• Oestrogen inhibits FSH — this prevents more than one egg from maturing at a time.
• Oestrogen stimulates LH — this positive feedback causes the LH surge that triggers ovulation.
• Progesterone inhibits FSH and LH — this prevents ovulation during the second half of the cycle and during pregnancy.

Exam Tip: The interaction between oestrogen and LH is an example of positive feedback — a rising level of oestrogen causes an even bigger increase in LH. This is unusual; most hormone interactions are negative feedback.

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Pregnancy and Hormones

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

• The developing embryo produces a hormone called hCG (human chorionic gonadotropin).
• hCG maintains the corpus luteum, so it continues producing progesterone.
• Progesterone keeps the uterus lining thick and intact, supporting the pregnancy.
• Eventually, the placenta takes over progesterone production.
• Pregnancy tests work by detecting hCG in the urine.

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Changes During Pregnancy