# Human Reproduction and Hormones
Human reproduction is controlled by hormones produced by the endocrine system. At GCSE, you need to understand how hormones control puberty, the menstrual cycle, and fertility. You also need to know about contraception methods and fertility treatments like IVF.
1. Puberty
Puberty is the stage of development when the reproductive system matures and a person becomes sexually mature.
Hormones Triggering Puberty
- The pituitary gland releases hormones (FSH and LH) that stimulate the sex organs
- In females: ovaries begin to produce oestrogen
- In males: testes begin to produce testosterone
Changes During Puberty
| Female Changes | Male Changes |
|---|---|
| Breasts develop | Voice deepens (breaks) |
| Hips widen | Shoulders broaden |
| Pubic and underarm hair grows | Pubic, underarm, and facial hair grows |
| Menstrual cycle begins (periods) | Testes and penis enlarge |
| Ovulation begins | Sperm production begins |
| Body fat redistributes | Increased muscle mass |
2. The Menstrual Cycle
The menstrual cycle is the monthly cycle of changes in the female reproductive system, lasting approximately 28 days. It is controlled by four key hormones.
The Four Hormones
| Hormone | Produced By | Function |
|---|---|---|
| FSH (Follicle-Stimulating Hormone) | Pituitary gland | Stimulates a follicle (containing an egg) to mature in the ovary; stimulates oestrogen production |
| Oestrogen | Ovaries | Causes the uterus lining to thicken; stimulates LH release (positive feedback); inhibits FSH |
| LH (Luteinising Hormone) | Pituitary gland | Triggers ovulation (release of the mature egg from the ovary) on approximately day 14 |
| Progesterone | Ovaries (corpus luteum) | Maintains the thick uterus lining; inhibits FSH and LH |
Stages of the Menstrual Cycle
Days 1–5: Menstruation
- The uterus lining breaks down and is shed (the period)
- Low levels of all hormones
Days 5–14: Follicular Phase
- FSH from the pituitary gland stimulates a follicle to develop in the ovary
- The developing follicle produces oestrogen
- Oestrogen causes the uterus lining to thicken (rebuild with blood vessels)
- Rising oestrogen stimulates a surge of LH
Day 14: Ovulation
- A spike in LH causes the mature egg to be released from the ovary
- The egg travels down the fallopian tube
Days 14–28: Luteal Phase
- The empty follicle becomes the corpus luteum, which produces progesterone
- Progesterone maintains the thick uterus lining
- Progesterone inhibits FSH and LH (prevents another egg from developing)
- If the egg is not fertilised, the corpus luteum degenerates, progesterone levels drop, and the uterus lining breaks down → menstruation begins again
- If the egg is fertilised, progesterone levels remain high to maintain the lining for pregnancy
3. Contraception
Contraception is the deliberate prevention of pregnancy. There are hormonal and non-hormonal methods.
Hormonal Methods
| Method | How It Works |
|---|---|
| Oral contraceptive pill (combined) | Contains oestrogen and progesterone; inhibits FSH so no egg matures; thickens cervical mucus |
| Progesterone-only pill (mini pill) | Thickens cervical mucus (prevents sperm reaching egg); may prevent ovulation |
| Contraceptive injection | Slow-release progesterone lasts 2–3 months; prevents ovulation |
| Contraceptive implant | Small rod inserted under skin; releases progesterone for up to 3 years |
| Contraceptive patch | Releases hormones through the skin; replaced weekly |
| Hormonal IUD (IUS) | Inserted into uterus; releases progesterone locally; thins uterus lining |
Non-Hormonal Methods
| Method | How It Works |
|---|---|
| Condom (male/female) | Barrier method; prevents sperm meeting egg; also prevents STIs |
| Diaphragm/cap | Covers the cervix; used with spermicide |
| Copper IUD (coil) | Inserted into uterus; copper is toxic to sperm; prevents implantation |
| Spermicide | Chemical that kills or immobilises sperm |
| Surgical sterilisation | Male (vasectomy) or female (tubal ligation); permanent |
| Abstinence | Not having sexual intercourse — 100% effective |
| Natural family planning | Monitoring body temperature and cervical mucus to avoid intercourse during fertile days |
Only condoms prevent both pregnancy AND sexually transmitted infections (STIs).
4. Fertility Treatments
Some couples have difficulty conceiving naturally. Medical treatments can help:
Fertility Drugs (Clomifene)
- Contains FSH and LH (or drugs that stimulate their production)
- Stimulates the ovaries to produce eggs
- Used when a woman is not ovulating regularly
- Risk: multiple pregnancies (twins, triplets) if too many eggs are released
IVF (In Vitro Fertilisation)
IVF is the process of fertilising an egg outside the body:
- The woman is given FSH and LH to stimulate the ovaries to produce multiple eggs
- Eggs are collected from the ovaries using a needle guided by ultrasound
- Eggs are mixed with sperm in a dish in the laboratory ("in vitro" = in glass)
- Fertilisation occurs, and the resulting embryos are allowed to develop for a few days
- One or two embryos are implanted into the woman's uterus
- If successful, the embryo implants in the uterus lining and pregnancy begins
IVF — Advantages and Disadvantages
| Advantages | Disadvantages |
|---|---|
| Allows infertile couples to have children | Emotionally and physically stressful |
| Can screen embryos for genetic disorders | Expensive (may not be funded by NHS) |
| Success rates are improving | Low success rate per cycle (~25-30%) |
| Unused embryos can help research | Ethical concerns about unused embryos (destroyed or frozen) |
| Risk of multiple pregnancies | |
| Hormonal treatment has side effects |
Worked Example
Question: Explain how the oral contraceptive pill prevents pregnancy. (3 marks)
Solution:
The combined oral contraceptive pill contains oestrogen and progesterone. These hormones inhibit FSH production from the pituitary gland. Without FSH, no follicle matures in the ovary and no egg is produced — so ovulation does not occur. The hormones also cause the cervical mucus to thicken, making it harder for sperm to reach any egg. Without ovulation, fertilisation cannot take place.
Practice Questions
- Name the four hormones involved in the menstrual cycle and state where each is produced. (4 marks)
- Describe what happens during ovulation and which hormone triggers it. (2 marks)
- Compare hormonal and non-hormonal methods of contraception. Give one example of each. (4 marks)
- Describe the stages of IVF treatment. (4 marks)
- Explain the role of progesterone in the menstrual cycle. (3 marks)
Answers
Want to check your answers and get step-by-step solutions?
Exam Tips
- Know the order: FSH → egg matures → oestrogen rises → LH surge → ovulation → progesterone maintains lining.
- The combined pill contains both oestrogen and progesterone — don't say "the pill contains FSH."
- For IVF questions, always mention: hormone stimulation → egg collection → fertilisation in lab → embryo implantation.
- Be prepared to discuss ethical issues around IVF and contraception.
Frequently Asked Questions
What happens if the egg IS fertilised?
If fertilised, the embryo implants in the uterus lining. The placenta develops and produces progesterone (and later other hormones) to maintain the uterus lining throughout pregnancy. Menstruation does not occur.
Why might IVF not work?
The embryo may fail to implant in the uterus lining. The eggs may not fertilise. The hormonal treatment may not produce enough eggs. The success rate per cycle is approximately 25-30%, so multiple cycles may be needed.
Is the contraceptive pill 100% effective?
No method except abstinence is 100% effective. The combined pill is over 99% effective when taken correctly. However, missing pills, illness (vomiting), or certain medications can reduce its effectiveness.
Summary
- Puberty is triggered by hormones: oestrogen (females) and testosterone (males).
- The menstrual cycle (~28 days) is controlled by FSH, oestrogen, LH, and progesterone through feedback loops.
- Ovulation (day 14) is triggered by an LH surge.
- Contraception can be hormonal (pill, injection, implant) or non-hormonal (condoms, IUD, sterilisation).
- IVF involves hormone stimulation, egg collection, lab fertilisation, and embryo implantation.