An embryo that has continued to grow for about 5 or 6 days is referred to as a blastocyst. Recent advances in technology have allowed for the successful culture of embryos to the blastocyst stage. In an IVF program embryos are usually transferred to the uterus at the cleavage cell stage (on the second or third day after egg retrieval) or at the blastocyst stage (on the fifth or sixth day after egg retrieval).
Blastocysts have a higher chance of implanting than earlier stage embryos. Many embryos stop growing at the four to eight cell stage, usually because of some inherent problem. Therefore, fewer embryos will have the ability to grow to the blastocyst stage. Those that successfully reach the blastocyst stage are more developmentally competent than earlier stage embryos. Therefore blastocyst culture does not turn an embryo that wont get someone pregnant into one that will, but it does allow us to identify those embryos that are more likely to result in a pregnancy.