Embryos can be transferred to the uterus any time after fertilization has occurred. Embryos have mostly been replaced day 2 or day 3 into the uterus in the past, but with improving culture conditions, more embryos are developing into blastocysts by day 5. The blastocyst is the first stage of embryo differentiation, its cells having separated into two distinct cell-lines, the inner cell mass and the trophectoderm. The blastocyst stage is the final stage of development before the embryo can hatch and implant in the uterus. In theory, blastocyst culture offers the possibility of a higher pregnancy rate mainly due to better embryo selection. Many embryos may have chromosome abnormalities. These abnormalities stop the embryo developing much beyond the 8-cell stage, or day 3 of development after egg collection. Blastocyst formation may therefore be a good indication of a high pregnancy potential embryo. Blastocyst culture may not be appropriate in all patients to ensure an optimal chance for pregnancy. Your specialist will discuss the advantages and disadvantages of blastocyst culture before the start of your treatment.