A blastocyst is an embryo that has developed for 5 days before being transferred back into the patient. Many patients undergoing IVF and ICSI treatment have more embryos available than are required for transfer. In order to select the best embryos for transfer and thus improve the outcome of treatment, it is sometimes desirable to culture the embryos past day 2 or day 3, especially when considering a Single Embryo Transfer (SET).
This extended culture allows embryos to develop into blastocysts and gives the embryologist the opportunity to select the best embryo(s) which are more likely to implant. Selection of embryo(s) for transfer on day 5 allows the transfer of fewer embryos reducing the risk of multiple pregnancies.