Implantation: Embryo Attachment to Uterus – Process and Timeline

Implantation is the process by which the blastocyst attaches to and invades the endometrial lining, establishing pregnancy.

Timeline & Stages

1. Fertilization (day 0): Sperm + oocyte → zygote in fallopian tube.
2. Cleavage (days 1–3): 2-cell → 4-cell → 8-cell → morula (16–32 cells).
3. Blastocyst formation (day 4–5): Trophoblast (outer) + inner cell mass + blastocoel (fluid cavity).
4. Hatching (day 5–6): Blastocyst escapes zona pellucida.
5. Apposition/Adhesion (day 6–7): Trophoblast contacts receptive endometrium (window of implantation).
6. Invasion (day 7–12): Trophoblast penetrates stroma, forms lacunae, establishes uteroplacental circulation.
7. Placentation: Chorionic villi develop; hCG production begins (detectable ~day 9–10 post-fertilization).

Clinical Relevance

Failure causes early pregnancy loss. Ectopic implantation (tube, ovary, abdomen) is life-threatening. ART optimizes endometrial receptivity for transfer timing.