Culture of human spare preembryos: association between blastocyst formation and pregnancy.

The rate of blastocyst development was studied in cultures of human fertilized eggs. A total of 195 spare embryos from 86 cycles in 73 in vitro fertilization (IVF) patients resulted in a total of 77 expanded blastocysts, corresponding to an overall blastocyst development of 39%. In the group of patients where pregnancy occurred, at least one of the spare embryos developed into the blastocyst stage in 100% of the patients, while the corresponding figure for nonpregnant patients was 53%. Superficially these results seem to indicate that blastocyst development could be of decisive importance for the success rate in IVF. A detailed analysis revealed, however, that this is not the case as long as in vitro culture before replacement is limited to 2-3 days.