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.