Blastocyst or cleavage-stage embryo transfer?

Key issue•Does blastocyst embryo transfer offer benefit over cleavage-stage embryo transfer in assisted conception treatment?What do we know?•Following natural conception, the embryo is traversing the fallopian tube at cleavage stage; it is in the uterus at blastocyst stage.•Embryo development to cleavage stage occurs under maternal genomic control.•To reach blastocyst stage, embryos need to develop under their own genomic control.•Selection of embryos, in order to replace the embryo or embryos with maximal viability, has presented a challenge.What do we think we know?•Implantation rates of blastocysts tend to be higher than those for cleavage-stage embryos. Should not the chance of success with a blastocyst culture and transfer policy be higher than that with cleavage-stage embryo transfer?•It might be possible to select couples who would benefit from blastocyst culture on the basis of milestones at certain stages of an in-vitro fertilization (IVF) cycle.•It might it be possible for clinics to reduce the multiple pregnancy rate and maintain the pregnancy rate by employing blastocyst culture.What do we not know?•Whether a policy of blastocyst culture offers genuine advantages by either increasing the chance of success of IVF/intracytoplasmic sperm injection or reducing the chance of multiple pregnancy.•Which couples, if any, benefit from blastocyst culture.