Embryo selection. Day 5 (blastocyst) culture and transfer allows the selection of the embryos most likely to implant and produce pregnancy. Most women can transfer one less embryo on day 5, reducing the chances of multiple gestation (twins, triplets). Pacific fertility center in san francisco has an outstanding program in day 5 culture and transfer, and elective single embryo transfer, to reduce risk at ivf.
Better Embryo. Great question. The longer an embryo grows the healthier the embryo. Often times genetically abnormal embryos stop growing after 3 or 4 days so a blastocyst has a higher chance of being genetically normal and thus resulting in a healthy pregnancy.