Which statement best defines a blastocyst and the typical timing of transfer in IVF?

• A. A blastocyst is a fertilized egg immediately after fertilization; transfer day 1.
• B. A blastocyst is an embryo cultured to day 5-6 with about 100 cells; transfers commonly on day 5.
• C. A blastocyst is six weeks after fertilization; transfer day 42.
• D. A blastocyst is any embryo; transfer day 3.

Answer: (B)

A blastocyst is the embryo that has developed to about day 5–6 after fertilization, containing roughly 100 cells and structured with an inner cell mass and a surrounding trophoblast that forms the placenta. Culturing to this stage lets clinicians observe embryo quality and viability more reliably than earlier stages, so the best candidates for transfer can be selected. Transferring the blastocyst on day 5 takes advantage of a well-matched window of uterine receptivity, improving implantation chances while often enabling single-embryo transfers. The other descriptions mix up developmental stages or timing: a fertilized egg right after fertilization is a zygote (not a blastocyst), day 3 is still a cleavage-stage embryo, and six weeks after fertilization is far beyond embryo development.