Embryo transfer is the process of putting embryos into the uterus via a catheter.  Embryo transfer can take place on day 2, day 3 or day 5 after egg collection.  A blastocyst is a stage of embryo development usually reached by day 5 or 6.  Leaving embryos until the blastocyst stage aids embryo selection but is only used when the embryo quality is good and it is difficult to choose the best embryo to transfer. The clinical pregnancy rate for all ages undergoing IVF are approximately 30% for day 2/3 transfers and 50% for blastocyst transfer.  However, it is important to understand that blastocyst transfer is not appropriate for every patient going through IVF and the results reflect a selected population of patients.