B16: Reproduction and Hormones
Human reproduction, hormones and contraception
📋 Key Concepts
Reproduction and Hormones: The menstrual cycle is controlled by four main hormones. These hormones can be used in contraception and in fertility treatments such as IVF.
Key Terms
- menstrual cycle
- FSH (follicle-stimulating hormone)
- LH (luteinising hormone)
- oestrogen
- progesterone
- ovulation
- contraception
- IVF (in vitro fertilisation)
- uterus lining
- follicle
📝 The Menstrual Cycle
The menstrual cycle is a roughly 28-day cycle in which an egg is released from an ovary and the uterus lining is prepared for a possible pregnancy. If the egg is not fertilised, the uterus lining breaks down (menstruation/period).
| Hormone | Produced by | Function |
|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Causes an egg to mature in the ovary (inside a follicle). Stimulates the ovaries to produce oestrogen. |
| LH (luteinising hormone) | Pituitary gland | Triggers ovulation (release of the mature egg from the ovary). Stimulates the remains of the follicle to develop into a corpus luteum. |
| Oestrogen | Ovaries | Builds up and maintains the uterus lining. Inhibits further FSH production (negative feedback). Stimulates LH release. |
| Progesterone | Ovary (corpus luteum) | Maintains the uterus lining during pregnancy. Inhibits both FSH and LH release (prevents another egg from being released). |
Hormone interactions:
- FSH stimulates oestrogen production
- Oestrogen inhibits FSH (negative feedback) and stimulates LH
- LH triggers ovulation
- Progesterone inhibits both FSH and LH
- If progesterone levels fall (no pregnancy), the uterus lining breaks down and the cycle restarts
Example 1
Explain why FSH levels are high at the start of the menstrual cycle but fall later.
Solution:
At the start of the cycle, FSH is released by the pituitary gland to stimulate egg maturation and oestrogen production. As oestrogen levels rise, oestrogen inhibits FSH production (negative feedback), so FSH levels fall. This ensures only one egg matures at a time.
Example 2
A woman has low levels of LH. Explain what effect this would have on the menstrual cycle.
Solution:
LH triggers ovulation (the release of a mature egg from the ovary). With low LH levels, ovulation would not occur, so no egg would be released. This would mean the woman could not become pregnant naturally because there is no egg available for fertilisation.
📝 Contraception
Contraception prevents pregnancy by preventing ovulation, preventing fertilisation, or preventing implantation of a fertilised egg.
| Method | How It Works | Advantages | Disadvantages |
|---|---|---|---|
| Combined pill (oral contraceptive) | Contains oestrogen and progesterone. Inhibits FSH production so no egg matures. | Over 99% effective if taken correctly. Can reduce period pain. | Must be taken daily. Possible side effects (headaches, nausea, blood clots). Does not protect against STIs. |
| Progesterone-only pill / injection / implant | Progesterone inhibits FSH and LH (prevents ovulation). Also thickens cervical mucus to block sperm. | Effective. Long-acting options (injection lasts 3 months, implant lasts 3 years). | Side effects possible. Does not protect against STIs. Fertility may take time to return after stopping. |
| Condom (barrier method) | Prevents sperm from reaching the egg. Physical barrier. | Protects against STIs. No side effects from hormones. Easily available. | Less effective than hormonal methods if not used correctly (about 98%). |
| IUD (intrauterine device) | Prevents implantation of a fertilised egg. Hormonal IUDs also release progesterone. | Long-lasting (5-10 years). Very effective. | Must be fitted by a doctor. May cause heavier periods. Does not protect against STIs. |
| Spermicide | Kills or disables sperm. | Easy to use. No side effects from hormones. | Not very effective alone (typically used with other methods). |
Example 3
Explain how the combined oral contraceptive pill prevents pregnancy.
Solution:
The combined pill contains oestrogen and progesterone. Oestrogen inhibits FSH production by the pituitary gland. Without FSH, no egg matures in the ovary, so ovulation does not occur. Progesterone also inhibits FSH and LH, further preventing egg maturation and ovulation. Without an egg being released, fertilisation cannot occur.
📝 IVF (In Vitro Fertilisation)
IVF is a fertility treatment used when a woman cannot conceive naturally. It involves stimulating the ovaries to produce multiple eggs, collecting them, fertilising them in a laboratory, and implanting embryos into the uterus.
IVF process:
- Give the woman FSH and LH to stimulate the ovaries to produce multiple mature eggs (superovulation)
- Collect the eggs from the ovaries
- Fertilise the eggs with sperm in a laboratory (in vitro = "in glass")
- Allow the fertilised eggs to develop into embryos
- Implant 1-2 embryos into the woman's uterus
- If successful, the embryo implants in the uterus lining and pregnancy begins
Ethical issues with IVF:
- IVF is expensive and not always available on the NHS — access may be limited
- Multiple embryos are often created but not all are implanted — some may be destroyed, raising ethical concerns about the right to life
- Multiple births (twins/triplets) are more likely with IVF, which carry higher health risks for mother and babies
- IVF success rates are not 100% — the emotional and physical stress of failed cycles can be significant
- Some religious groups object to IVF because it involves fertilisation outside the body and the potential destruction of embryos
Example 4
A couple cannot conceive because the woman does not ovulate. Explain how IVF could help them.
Solution:
In IVF, the woman is given FSH and LH hormones to stimulate her ovaries to produce multiple mature eggs (overcoming the lack of natural ovulation). The eggs are then collected from her ovaries and fertilised with sperm in the laboratory. The resulting embryos are implanted into her uterus, where they can develop into a pregnancy. This bypasses the problem of natural ovulation failure.
❓ Practice Questions
Q1: Name the four hormones that control the menstrual cycle. For each hormone, state where it is produced and its main function.
Q2: Explain how oestrogen and progesterone act as negative feedback controls in the menstrual cycle.
Q3: Describe how the combined oral contraceptive pill prevents pregnancy.
Q4: Outline the steps involved in IVF treatment and explain why FSH and LH are given at the start.
Q5: Give two ethical issues associated with IVF treatment.
✅ Answers
- FSH — produced by pituitary gland, causes egg maturation and stimulates oestrogen production. LH — produced by pituitary gland, triggers ovulation. Oestrogen — produced by ovaries, builds up uterus lining and inhibits FSH. Progesterone — produced by corpus luteum (ovary), maintains uterus lining and inhibits FSH and LH.
- Oestrogen inhibits FSH production (prevents further egg maturation when an egg is already developing). Progesterone inhibits both FSH and LH (prevents another egg from being released while the uterus lining is being maintained). This negative feedback ensures only one egg matures and is released per cycle.
- The combined pill contains oestrogen and progesterone. Oestrogen inhibits FSH so no egg matures in the ovaries. Progesterone also inhibits FSH and LH, preventing ovulation. Without an egg being released, pregnancy cannot occur.
- Steps: (1) Give FSH and LH to stimulate the ovaries to produce multiple mature eggs (superovulation). (2) Collect eggs from the ovaries. (3) Fertilise eggs with sperm in the lab. (4) Allow embryos to develop. (5) Implant 1-2 embryos into the uterus. FSH and LH are given because the woman may not be producing enough naturally to stimulate ovulation — these hormones ensure multiple eggs mature and are released for collection.
- Any two from: some embryos are created but not all are used/implanted (may be destroyed, raising right-to-life concerns); IVF is expensive and not equally available; multiple births carry health risks; emotional stress of failed treatments; religious objections to fertilisation outside the body.
🎯 Exam Tips
- Learn all four hormones, where they are produced, and what they do — this is very commonly tested
- FSH and LH are produced by the PITUITARY GLAND; oestrogen and progesterone by the OVARIES
- Remember: oestrogen INHIBITS FSH; progesterone INHIBITS FSH and LH — these are negative feedback
- When asked about contraception, explain HOW it works at a hormonal level (not just "it prevents pregnancy")
- For IVF, always mention FSH and LH at the start to stimulate superovulation
- Don't confuse FSH with LH: FSH matures the egg, LH triggers ovulation (release)
- When discussing ethical issues, give balanced points and explain both sides
- The "in vitro" in IVF means "in glass" (in the lab), not inside the body
🔢 Maths Skills
Mathematical Skills
Interpreting menstrual cycle graphs: Graphs show four hormones (FSH, LH, oestrogen, progesterone) plotted against days of the cycle. Day 1 is the start of menstruation. Identify ovulation around day 14 where LH peaks sharply. Read hormone concentrations at specific days — FSH is highest around days 1–5, oestrogen rises from day 7 to a peak around day 12, LH surges at day 14, and progesterone rises after ovulation (day 15–28). Calculate the length of phases: follicular phase (days 1–13), ovulation (day 14), luteal phase (days 15–28).
⚠️ Common Misconceptions
Watch Out!
1. IVF always works. Wrong: IVF has a 100% success rate. Correct: IVF success rate is approximately 25–30% per cycle for women under 35, and decreases with age — multiple cycles may be needed.
2. The contraceptive pill is 100% effective. Wrong: the pill completely guarantees no pregnancy. Correct: the combined pill is over 99% effective with perfect use, but with typical use (occasionally missing pills) it is about 91% effective.
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how hormones control the menstrual cycle and discuss IVF ethics.
FSH, released by the pituitary gland, causes an egg to mature in the ovary and stimulates the ovaries to produce oestrogen. Oestrogen builds up the uterus lining, inhibits further FSH production (negative feedback), and stimulates LH release. LH, also from the pituitary gland, triggers ovulation (release of the egg) around day 14. After ovulation, progesterone maintains the uterus lining and inhibits both FSH and LH to prevent another egg maturing. If no embryo implants, progesterone falls, the uterus lining breaks down, and the cycle restarts. IVF raises ethical issues: surplus embryos may be destroyed, which some believe is unethical; it is expensive and not equally available; multiple births carry health risks; and some religious groups object to fertilisation outside the body. However, IVF gives infertile couples the chance to have a child, which is a significant benefit.
Mark scheme: 1 mark — FSH causes egg maturation and stimulates oestrogen; 1 mark — oestrogen builds uterus lining and inhibits FSH; 1 mark — LH triggers ovulation; 1 mark — progesterone maintains lining and inhibits FSH/LH; 1 mark — at least one ethical issue explained; 1 mark — balanced consideration (benefit vs concern).
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
The table shows IVF success rates by age: under 35 = 29%, 35–37 = 23%, 38–39 = 15%, 40–42 = 8%, over 42 = 3%. Describe the trend in the data and suggest biological reasons for the pattern. A 41-year-old woman is considering IVF. Using the data, evaluate whether she should proceed and what factors she should consider.
📝 Exam Questions by Topic
🎬 Video Resources
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