Methods for the culture of preimplantation human embryos evolved primarily from those used for mouse embryos. The initial method was usually culture in a single medium in microdrops of medium under oil for 2–3 days before transfer. Subsequently, extended culture over the whole preimplantation period was used. The debate at present is which system is best, a sequential series of media to accommodate changes in physiology and metabolism of the embryo from a 1-cell zygote to the differentiated blastocyst stage or a single-step culture regime using the same culture medium throughout the preimplantation period. Aspects of the advantages and disadvantages of these two culture systems will be discussed.
