Reproduction: Humans & Health
Placenta structure and exchange; harmful substances crossing the placenta; Extended antenatal care; sex chromosomes and determination; HIV/AIDS — transmission, immune system damage, AIDS development; gonorrhoea and syphilis; STI prevention; contraception methods; Extended contraceptive pill mechanism via FSH inhibition.
Antenatal and Postnatal Care
CORE EXTENDEDThe Placenta
The placenta is the organ that connects the developing foetus to the mother’s uterine wall. It allows exchange of substances between maternal and foetal blood without the two bloodstreams mixing.
| Substance | Direction of transfer | How transferred |
|---|---|---|
| Oxygen | Mother → foetus | Diffusion (maternal blood has higher O₂ concentration) |
| Glucose | Mother → foetus | Diffusion / facilitated diffusion |
| Amino acids | Mother → foetus | Active transport |
| Antibodies | Mother → foetus | Cross the placenta by selective transfer — provide passive immunity to newborn |
| Carbon dioxide | Foetus → mother | Diffusion (foetal blood has higher CO₂ concentration) |
| Urea | Foetus → mother | Diffusion (waste removed via mother’s kidneys) |
The placenta has a large surface area with thin walls between maternal and foetal blood supplies. Exchange occurs across this barrier — the two bloodstreams remain separate. This prevents the mother’s immune system from attacking the foetus and stops incompatible blood types from mixing.
Harmful Substances Crossing the Placenta
| Substance | Effects on foetus |
|---|---|
| Alcohol | Foetal alcohol syndrome — impaired brain development; low birth weight; physical abnormalities; lifelong disability |
| Nicotine / carbon monoxide (smoking) | Reduced oxygen to foetus (CO reduces haemoglobin’s O₂ carrying capacity); low birth weight; premature birth; increased miscarriage risk |
| Some drugs | Various teratogenic effects depending on drug; heroin causes newborn withdrawal symptoms (neonatal abstinence syndrome) |
| Rubella virus | Deafness, blindness, heart defects, brain damage in foetus — especially dangerous in first trimester |
Antenatal Care — Extended
Antenatal care is the medical care provided to a pregnant woman and her foetus before birth. Key components:
Use high-frequency sound waves to image the foetus. Check foetal growth, position, number of foetuses, and detect structural abnormalities (e.g. heart defects). Typically at 12 weeks (dating scan) and 20 weeks (anomaly scan).
Check maternal haemoglobin (anaemia screening); blood group and Rhesus factor; screening for infections (HIV, hepatitis B, rubella immunity); chromosomal abnormality screening markers.
Regular measurements of uterus size (symphysis-fundal height) and foetal movements. Poor growth or absent movements can indicate placental insufficiency or foetal distress.
Avoid alcohol, smoking, and illicit drugs; avoid certain foods (unpasteurised cheese, raw meat — listeria/toxoplasmosis risk); take folic acid supplements (reduces neural tube defects); avoid certain medications.
How does the foetus obtain oxygen from the mother during pregnancy?
- A. The foetus breathes amniotic fluid, which contains dissolved oxygen
- B. Maternal and foetal blood mix in the placenta, sharing oxygen
- C. Oxygen diffuses from maternal blood across the placenta into foetal blood
- D. The umbilical artery carries oxygenated blood from the mother directly to the foetus
Sex Determination
CORESex Chromosomes
Humans have 23 pairs of chromosomes. Pair 23 are the sex chromosomes:
Females: XX (two X chromosomes)
Males: XY (one X and one Y chromosome)
Sex is determined at fertilisation by which sex chromosome the sperm contributes (eggs always contain X; sperm contain either X or Y).
Parents: Female (XX) × Male (XY)
Gametes: X X or Y
Offspring: XX (female) or XY (male)
Ratio: 1 female : 1 male (50% probability each)
Which statement about sex determination in humans is correct?
- A. Females are XY and males are XX
- B. Sex is determined by the chromosome in the egg cell
- C. A Y-bearing sperm fertilising an egg produces a male offspring
- D. All offspring of a female are female because females only have X chromosomes
Sexually Transmitted Infections
CORE EXTENDEDHIV and AIDS
HIV (Human Immunodeficiency Virus) is a virus that attacks and destroys T-helper lymphocytes (CD4+ T cells), which are essential for coordinating the immune response.
AIDS (Acquired Immune Deficiency Syndrome) is the condition that develops when the immune system is so severely damaged by HIV that the body can no longer fight off infections and cancers that a healthy immune system would control — called opportunistic infections.
| Feature | HIV |
|---|---|
| Type of pathogen | Virus (retrovirus) |
| Transmission routes | Unprotected sexual intercourse; sharing needles (contaminated blood); mother to child (pregnancy, childbirth, breastfeeding); blood transfusion (in countries without screening) |
| Target cells | T-helper lymphocytes (CD4+ T cells) |
| Effect on immune system | Gradually destroys T-helper cells → immune system weakens → susceptible to opportunistic infections (e.g. pneumonia, tuberculosis, certain cancers) |
| Treatment | Antiretroviral therapy (ART) — suppresses viral replication, allowing immune system to recover; cannot cure HIV but allows near-normal life expectancy |
| Prevention | Condom use; not sharing needles; antiretroviral drugs (PrEP); blood screening; mother-to-child transmission prevention programmes |
Other STIs
| STI | Pathogen type | Symptoms | Treatment |
|---|---|---|---|
| Gonorrhoea | Bacterium (Neisseria gonorrhoeae) | Painful urination; discharge; often asymptomatic (especially in women) | Antibiotics (but antibiotic-resistant strains increasing) |
| Syphilis | Bacterium (Treponema pallidum) | Progressive stages: painless sore (chancre) → rash → organ damage if untreated | Antibiotics (penicillin) |
Consistent and correct use of condoms is the most effective barrier method preventing transmission of STIs including HIV, gonorrhoea, and syphilis. Condoms prevent contact between sexual fluids and mucous membranes, blocking pathogen transmission. Abstinence and partner reduction also reduce risk.
A person with HIV does not immediately develop AIDS. Which statement correctly explains why?
- A. HIV is not infectious until AIDS develops
- B. AIDS develops only after the immune system has been sufficiently damaged by HIV destroying T-helper lymphocytes
- C. AIDS is caused by a different virus that infects people who already have HIV
- D. The body can eliminate HIV before AIDS develops using antibiotics
Contraception
CORE EXTENDEDMethods of Contraception
| Method | How it works | Also prevents STIs? |
|---|---|---|
| Condom (male/female) | Physical barrier — prevents sperm reaching egg; also prevents STI transmission | ✓ Yes |
| Oral contraceptive pill | Contains oestrogen and/or progesterone — inhibits FSH release → no follicle development → no ovulation | ✗ No |
| Intrauterine device (IUD) | Device placed in uterus — prevents implantation; some release hormones that thicken cervical mucus | ✗ No |
| Diaphragm / cap | Physical barrier placed over cervix — prevents sperm reaching egg | ✗ No reliable protection |
| Abstinence | No sexual intercourse — 100% effective if maintained | ✓ Yes |
| Sterilisation | Vasectomy (males — sperm ducts cut/tied); tubal ligation (females — oviducts cut/tied); permanent | ✗ No |
How the Contraceptive Pill Works — Extended
The combined oral contraceptive pill contains synthetic oestrogen and progesterone. The high levels of these hormones:
1. Inhibit secretion of FSH from the pituitary gland (negative feedback).
2. Without FSH, no follicle develops in the ovary.
3. Without a follicle, no LH surge occurs, so no ovulation takes place.
4. Additionally, progesterone thickens cervical mucus (making it harder for sperm to penetrate) and keeps the uterus lining thin (less favourable for implantation).
The pill prevents fertilisation by stopping ovulation — it is a hormonal method with no physical barrier. It has no effect on the transmission of viruses (HIV) or bacteria (gonorrhoea, chlamydia) through sexual contact. Only condoms provide meaningful barrier protection against both pregnancy and STI transmission; diaphragm and cap do not provide reliable STI protection.
Which method of contraception also protects against sexually transmitted infections?
- A. Oral contraceptive pill
- B. Intrauterine device (IUD)
- C. Condom
- D. Sterilisation (vasectomy)
Comprehensive Practice Questions
Mixed questions across Topics 16.4–16.7.
A pregnant woman smokes cigarettes. Which effect on the foetus is directly caused by carbon monoxide in cigarette smoke?
- A. Increased risk of foetal alcohol syndrome
- B. Reduced oxygen delivery to the foetus because haemoglobin carries less oxygen
- C. Direct damage to foetal lung tissue
- D. Increased glucose transport across the placenta
(a) Describe two ways in which HIV can be transmitted from person to person. [2 marks]
(b) Explain how HIV causes AIDS. [3 marks]
(c) State two methods of contraception and explain how each prevents pregnancy. [4 marks]
- (a) Any two of: unprotected sexual intercourse; sharing contaminated needles; mother to child during pregnancy/birth/breastfeeding; contaminated blood transfusion [2 marks]
- (b) HIV infects and destroys T-helper lymphocytes [1 mark]; as T-helper cell numbers decline, the immune system weakens and cannot coordinate an effective immune response [1 mark]; the person becomes susceptible to opportunistic infections and cancers that a healthy immune system would control — this stage is AIDS [1 mark]
- (c) Any two, e.g. Condom: physical barrier that prevents sperm from entering the vagina / reaching the egg [2 marks]; Oral contraceptive pill: contains hormones (oestrogen/progesterone) that inhibit FSH release, preventing follicle development and ovulation [2 marks]
(a) Explain how the combined oral contraceptive pill prevents pregnancy, with reference to FSH, LH, and ovulation. [4 marks]
(b) Suggest two reasons why a pregnant woman is advised to avoid alcohol during pregnancy. [2 marks]
- The pill contains synthetic oestrogen and progesterone [1 mark]
- These hormones inhibit the release of FSH from the pituitary gland (negative feedback) [1 mark]
- Without FSH, no follicle develops in the ovary, so no LH surge occurs [1 mark]
- Without an LH surge, ovulation does not occur → no egg is available to be fertilised [1 mark]
- Alcohol crosses the placenta and can cause foetal alcohol syndrome — impaired brain development and physical abnormalities in the foetus [1 mark]
- Alcohol can lead to low birth weight / premature birth / increased miscarriage risk [1 mark]
High-Frequency Mistakes — Topics 16.4–16.7
- 🔄Saying maternal and foetal blood mix in the placentaThe two bloodstreams remain completely separate throughout pregnancy. Exchange occurs by diffusion and active transport across the thin placental barrier. Mixing would cause the mother’s immune system to attack the foetus. Always say "diffuses across" or "transported across" the placenta — not "enters the mother’s blood."
- 💊Saying antibiotics can treat HIVHIV is a virus. Antibiotics only kill bacteria. HIV is treated with antiretroviral drugs (ART) which suppress viral replication. This is a very common error — reinforced by the Topic 15 lesson that antibiotics don’t affect viruses.
- 👶Confusing the umbilical artery and umbilical veinThe umbilical vein carries oxygenated blood FROM the placenta TO the foetus. The umbilical arteries carry deoxygenated blood FROM the foetus TO the placenta. This reverses the usual artery/vein convention (arteries usually carry blood away from the heart — here they carry deoxygenated blood away from the foetus).
- 📄Ext: Saying the pill prevents implantation (main mechanism)The primary mechanism of the combined pill is preventing ovulation by inhibiting FSH (and thus follicle development and the LH surge). The secondary effects (thickening cervical mucus, thinning uterus lining) are additional but not the main mechanism. Questions asking "how does the pill prevent pregnancy" should focus on the FSH → no ovulation chain.
- ⚧Saying sex is determined by the motherThe mother always contributes an X chromosome. The father contributes either X (→ XX = female) or Y (→ XY = male). Sex is therefore determined by the father’s sperm, not by the mother’s egg.
- 💉Saying AIDS is a virusAIDS is a syndrome — a collection of symptoms and conditions resulting from severe immune deficiency. It is caused by HIV (the virus) destroying T-helper cells. HIV is the virus; AIDS is the clinical condition that results from advanced HIV infection.
Highest-yield Core items: placenta exchange table (what crosses, which direction, and how — diffusion vs active transport); harmful substances crossing the placenta (alcohol, CO from smoking, drugs, rubella); sex determination (XX female, XY male — father’s sperm determines sex); HIV definition, transmission routes, effect on T-helper cells, AIDS development; condom as the only contraceptive method protecting against both pregnancy and STIs. For Extended: contraceptive pill mechanism (oestrogen + progesterone → inhibit FSH → no ovulation → cannot get pregnant); antenatal care components. The placenta exchange table and HIV/AIDS mechanism are highly reliable Paper 4 targets.