Coculture techniques on monolayers cells increase the rate of human blastocysts development. The selection of these heavier and more expensive methods should be reserved for cases which this work specifies. The rate of achieved pregnancies seems increased, embryo quality being, at this stage of later development, better estimated, from which a more objective selection. The window of endometrial receptivity to blastocysts implantation seems later than the one defined by the 2/8 cells stage. Last but not least, the cryopreservation of high quality blastocysts favours a high rate of recovery on defreezing.