Approximately how many days after fertilization does the blastocyst form?

• A. 5-6 days after fertilization.
• B. 1-2 days after fertilization.
• C. 10-12 days after fertilization.
• D. 20-25 days after fertilization.

Answer: (A)

Blastocyst formation occurs after a short series of cell divisions following fertilization. By about five to six days after fertilization, fluid enters the structure and it becomes a blastocyst, with an outer trophoblast layer and an inner cell mass. Implantation into the uterine wall begins around day six to seven and continues over the next several days. So the blastocyst forms approximately five to six days after fertilization. The other options fall outside this window: one to two days is too early for blastocyst formation, 10–12 days is after the blastocyst stage has formed and implantation is underway, and 20–25 days is well into later embryogenesis after implantation.