In order to implant the embryo, it must form a blastocyst, expand and leave the zona pellucida (the outer layer that surrounds it), so implantation does not begin until 6-7 days after fertilization. If the transfer is made on day 3, it would take 2-3 more days for the time to begin implantation to arrive, and if it is made on day 5, 1 or 2 more days.