5. Implantation

5.1 Timeline

  • Begins Day 6–7 after fertilization.
  • Completed by Day 12.

5.2 Stages of Implantation

  • Apposition → Adhesion → Invasion

5.3 Trophoblast Differentiation

  • Cytotrophoblast: inner proliferative layer
  • Syncytiotrophoblast: invasive outer layer that:
    • Secretes hCG
    • Invades endometrium
    • Establishes maternal blood flow

5.4 Decidual Reaction

  • Endometrial cells enlarge, store nutrients.
  • Protects embryo from maternal immune system.