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D3.1.18 (HL)—Placenta role

The placenta exchanges materials between maternal and fetal blood, prevents blood mixing and secretes hormones that help maintain pregnancy in mammals.

Syllabus
First assessment 2025
Objective
D3.1.18
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper1
Typical marks1–8

Common command terms

  • Explain
  • Identify

Recent exam appearances

May 2023Paper1 ["HL"] · TZ240[ 1 ]D3.1.18 (HL)—Placenta role
May 2017Paper1 ["HL"] · TZ140[ 1 ]D3.1.18 (HL)—Placenta role
November 2014Paper2 ["HL"] · TZ08(c)[ 8 ]D3.1.18 (HL)—Placenta role
May 2013Paper2 ["HL"] · TZ28(c)[ 8 ]D3.1.18 (HL)—Placenta role
Practice this objective

Coverage 2013–2023 · Updated 16 Jul 2026

The Placenta Exchanges Materials between Mother and Fetus

HL only

The placenta supports foetal development by exchanging materials across placental villi while maternal and foetal blood normally remain separate.

Numerous villi provide a large surface area and a short exchange path. Oxygen, nutrients and maternal antibodies move toward foetal blood; carbon dioxide, urea and other wastes move toward maternal blood; placental hormones help maintain pregnancy.

The placenta allows the foetus to remain and develop inside the uterus to a later stage than in mammals without a placenta, while maternal physiology supplies continuous exchange.

Oxygen diffuses down its concentration gradient from maternal blood across a villus into foetal capillaries, while carbon dioxide moves in the opposite direction.

Large surface area does not mean the two blood supplies mix. The placental barrier is selective but not complete; some drugs and pathogens can cross.

Placenta role

HL only

Assessment in practice

1–8 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Identify.

Command terms

Explain / Identify

What earns marks

Build the answer around this relationship: Placental villi increase surface area for maternal-fetal exchange.

Representative question

Question 1

[Maximum number: 8]

Explain how the structure and functions of the placenta maintain pregnancy.

Retrieve the HL Reproduction Route

HL only

HL D3.1 is about ordered mechanisms and evidence judgment: endocrine control, gametogenesis contrast, one-sperm fertilization, early embryo stages, hCG detection, placental exchange, birth feedback, and HRT evaluation.

  • GnRH drives pituitary FSH/LH and sex-hormone production
  • spermatogenesis gives four sperm; oogenesis gives one ovum and polar bodies
  • acrosome entry, cortical block, cleavage, blastocyst, endometrium
  • hCG, placenta, childbirth feedback, and HRT risk-benefit evaluation

HL Human Reproduction

HL only

HL D3.1 moves from reproductive events into control and evidence. The strongest answers keep sequences in order: endocrine axis at puberty, gametogenesis outcomes, fertilization blocks, blastocyst implantation, hCG testing, placental exchange, childbirth feedback, and HRT evaluation.

  • Explain HL mechanisms in order, especially hormone pathways and early-development stages.
  • Compare gametogenesis and fertilization mechanisms using precise structural terms.
  • Evaluate HRT by weighing symptom benefits against risks and evidence quality.

Concept essentials

  • Placental villi increase surface area for maternal-fetal exchange.
  • Maternal and fetal blood flow close together but do not normally mix.
  • The placenta secretes hormones that help maintain pregnancy.