Topic 15 covers human sexual reproduction. It starts with puberty and the sex hormones that trigger it, then the structures and functions of the male and female reproductive systems. It goes on to the menstrual cycle, fertilisation and implantation, how the foetus is supported during pregnancy, and sexually transmitted infections including HIV/AIDS.
Schools test this topic in both MYE and EOY papers. It appears as quick MCQs (identify the structure, name the hormone), as calendar and graph questions on the menstrual cycle, and as structured questions where you must name a part and state its function. Marks are most often lost through vague wording, so learn each organ together with its job and use the exact terms.
What these notes cover
- •Puberty and sex hormones
- •Primary and secondary sexual characteristics
- •The male reproductive system
- •The female reproductive system
- •The menstrual cycle, day by day
- •Ovulation and the fertile period
- •Fertilisation, zygote and implantation
- •Sperm versus egg: structure and function
- •Heredity and twins
- •Pregnancy: placenta, umbilical cord and amniotic sac
- •STIs and HIV/AIDS
- •Common mistakes and exam technique
Key definitions
- Puberty
- The stage of life during which the reproductive organs mature and a child’s body develops into an adult’s. It is triggered by sex hormones: testosterone in males, and oestrogen and progesterone in females.
- Ovulation
- The release of a mature egg from an ovary into the oviduct (Fallopian tube). It happens on about day 14 of a 28-day cycle, and usually only one egg is released per cycle.
- Fertilisation
- The fusion of the nucleus of a sperm with the nucleus of an egg to form a single cell called a zygote. In humans, fertilisation takes place in the oviduct.
- Implantation
- The attachment of the embryo to the thickened uterine lining. The embryo is the ball of cells formed after the zygote divides repeatedly.
- HIV and AIDS
- HIV (Human Immunodeficiency Virus) is the virus that attacks and destroys cells of the body’s immune system. AIDS (Acquired Immune Deficiency Syndrome) is the late stage of HIV infection, when the body can no longer fight off other infections.
Key formulas
Sperm production temperature
T(scrotum) ≈ T(core body) − 2 °C ≈ 35 °C
The scrotum keeps the testes about 2 °C below core body temperature (about 35 °C instead of 37 °C). This lower temperature is needed for sperm production.
Calendar day counting
date of day n = date of day 1 + (n − 1) days
Day 1 is the first day of menstrual bleeding, not the last. Day 14 is 13 days after day 1, not 14 days after.
Estimating ovulation (28-day cycle)
ovulation date ≈ first day of period + 13 days (about day 14)
28 days is an average, and cycle length varies. Always write “about day 14”.
Fertile period (28-day cycle)
fertile period ≈ (day 14 − 3 days) to (day 14 + 2 days) ≈ days 11 to 16
Sperm survive about 3–4 days in the female body, and the egg survives about 1–2 days after ovulation.
First day of next period (28-day cycle)
first day of next period ≈ first day of period + 28 days
The next cycle starts on day 29, which is day 1 of the new cycle.
Exchange at the placenta
O₂ + dissolved nutrients: mother → foetus; CO₂ + wastes: foetus → mother
Exchange is by diffusion across the placenta, and the two bloodstreams never mix.
Worked example
Amirah records the first day of her menstrual period as Tuesday 8 September. Assuming her cycle is a regular 28-day cycle, estimate (a) her date of ovulation, (b) her fertile window, (c) the first day of her next period.
- 1Day 1 of the cycle is 8 September, so any day n of the cycle falls (n − 1) days after 8 September.
- 2(a) Ovulation occurs on about day 14: day 14 = 8 Sep + 13 days = 21 September.
- 3(b) The fertile window is about days 11 to 16: day 11 = 8 Sep + 10 days = 18 September, and day 16 = 8 Sep + 15 days = 23 September.
- 4So she is fertile from about 18 September to about 23 September.
- 5(c) The next cycle starts on day 29, i.e. 28 days after day 1: 8 Sep + 28 days = 6 October.
- 6Every answer should carry the word “about”, since real cycles vary in length.
Watch out: Off-by-one error: day 14 is 13 days after day 1, not 14 days after. Counting from the last day of bleeding instead of the first also shifts every answer by about 5 days.
Common mistakes
Placing fertilisation in the uterus
Wrong: Fertilisation happens in the uterus.
Right: Fertilisation (fusion of the sperm nucleus with the egg nucleus) takes place in the oviduct. The embryo then implants in the uterus.
Saying only blood is shed at menstruation
Wrong: During menstruation, blood is shed.
Right: The uterine lining breaks down and, together with some blood, is discharged through the vagina. The lining is the main thing lost.
Putting mosquito bites on the HIV transmission list
Wrong: HIV can be spread by mosquito bites.
Right: Mosquitoes inject saliva, not blood, and HIV cannot survive or multiply inside a mosquito. HIV spreads only through infected body fluids: sexual contact, infected blood or needles, and from mother to child.
Questions students ask
Where does fertilisation take place in humans?
Fertilisation takes place in the oviduct (Fallopian tube), not the uterus. The zygote divides repeatedly as it moves down the oviduct. The resulting embryo then implants in the thickened uterine lining.
Why must the mother’s blood and the foetus’s blood not mix?
At the placenta, the two bloodstreams flow close together and exchange substances by diffusion, but they never mix. The mother and foetus may have different blood groups, so mixed blood could clump together (agglutinate). The mother’s blood is also at a much higher pressure, which would damage the foetus’s delicate blood vessels.
What is the difference between identical and fraternal twins?
Identical twins form when one egg is fertilised by one sperm and the single zygote splits into two embryos. They have exactly the same genetic material and are always the same sex. Fraternal twins form when two eggs are each fertilised by a different sperm, so they are genetically like ordinary siblings and can be different sexes.
Which STIs can be cured by antibiotics?
Gonorrhoea and syphilis are caused by bacteria, so they can be treated and cured with antibiotics, especially if treated early. HIV is a virus, and antibiotics have no effect on viruses. Any MCQ option that includes HIV or AIDS is wrong.
In the full PDF
- •Labelled diagrams of the male and female reproductive systems and of the foetus in the uterus
- •The lining-thickness graph for the menstrual cycle, with how to read it in an exam
- •Further worked examples with model answers and mark allocations: scrotum temperature, sperm vs semen, sperm structure, twins, the placenta data question and HIV MCQs
- •All ten misconceptions to un-learn, plus the full lists of common mistakes for each section
- •The exam-technique table comparing weak answers with full-credit phrasing
- •Section summaries and the five takeaways for the exam hall