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Learn5090Paper 1 · Multiple ChoiceDevelopment of organisms and continuity of lifeHuman reproduction
Development of organisms and continuity of life · Subtopic 2 of 2

Human Reproduction

Sperm are made in the testes; eggs are released from the ovaries. Fertilisation happens in the oviduct; the zygote divides into an embryo that implants in the uterus lining. The placenta lets oxygen and nutrients diffuse from mother to fetus and CO₂ and urea back, without the bloods mixing; amniotic fluid cushions the fetus. The menstrual cycle (about 28 days) is controlled by hormones: oestrogen rebuilds the uterus lining, LH triggers ovulation around day 14, progesterone maintains the lining. STIs such as HIV spread through sexual contact and can be prevented with condoms.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the male and female reproductive systems and gametes.
  • Describe fertilisation, implantation and the role of the placenta.
  • Describe the menstrual cycle and its hormones.
  • Describe how STIs such as HIV are spread and controlled.
Pause and think

A fetus needs oxygen but can't breathe in the uterus. How does it get it?

Section 2 of 14

Core idea

Start hereNine months of exchange

For nine months, a developing baby gets everything through one organ the size of a dinner plate: the placenta.

The big idea

Gametes meet in the oviduct; the embryo implants in the uterus. Hormones control the monthly cycle; the placenta supplies the fetus without mixing blood.

1. The menstrual cycle

A 28-day cycle

Oestrogen rebuilds, progesterone maintains.

Worked exampleThe placenta

List substances crossing the placenta in each direction.

Goal
Two lists.
Strategy
Needs vs wastes.
  1. to fetus: O2, glucose, amino acids, antibodies\text{to fetus: O}_2,\ \text{glucose, amino acids, antibodies}
    Supplies.
  2. to mother: CO2, urea\text{to mother: CO}_2,\ \text{urea}
    Wastes.
AnswerExchange by diffusion; the two blood supplies don't mix.
✓ CheckAlcohol, nicotine and some viruses can also cross ✓.
Common misconceptionBlood mixing Maternal and fetal blood never mix: they could have different blood groups and the fetal blood pressure is lower.
Pause and think

Why does a fetus need antibodies from its mother?

Summary

  • Fertilisation in oviduct.
  • Placenta: exchange without mixing.
  • Oestrogen builds; progesterone maintains.
Key words
Ovulation
Release of an egg from an ovary.
Implantation
Embryo embedding in the uterus lining.
Placenta
Organ of exchange between mother and fetus.
HistoryIVF. Louise Brown, born in England in 1978, was the first baby conceived by in vitro fertilisation. Over 10 million people have since been born through IVF.
Fun factSize difference. A human egg is about 0.1 mm across, just visible to the eye; a sperm head is about 0.005 mm, twenty times smaller.
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