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D3.1.16 (HL)—Blastocyst and implantation

A blastocyst is an early embryo that has developed into a hollow ball of cells before implantation in the uterine lining.

Syllabus
First assessment 2025
Objective
D3.1.16
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2012
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Recent exam appearances

May 2012Paper1 ["HL"] · TZ140[ 1 ]D3.1.16 (HL)—Blastocyst and implantation
Practice this objective

Coverage 2012–2012 · Updated 16 Jul 2026

The Blastocyst Must Implant in the Uterine Lining

HL only

After early cleavage, the embryo forms a blastocyst whose outer cells attach to and invade the uterine lining during implantation.

The inner cell mass forms the embryo; the outer trophoblast helps attachment and later placenta formation. Implantation connects embryonic development to maternal support.

Distinguish cleavage; blastocyst formation; implantation; and later organ development in sequence.

A blastocyst reaches the uterus and its trophoblast attaches to the endometrium, allowing implantation to begin.

Fertilization in the oviduct is not implantation; implantation requires later attachment to the uterine lining.

Blastocyst and implantation

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: A blastocyst is an embryo, not an unfertilized egg.

Representative question

Question 1

[Maximum number: 1]

What is a blastocyst?

A

An unfertilized egg surrounded by follicle cells

B

An unfertilized egg cell expelled by menstruation

C

The follicle when it has swelled up with fluid

D

The embryo when it has become a hollow ball of cells

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

  • A blastocyst is an embryo, not an unfertilized egg.
  • The blastocyst is a hollow ball of cells.
  • Blastocyst formation occurs before implantation in the uterus.