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.