Prospective randomized study of two cryopreservation policies avoiding embryo selection: the pronucleate stage leads to a higher cumulative delivery rate than the early cleavage stage

Objective: To compare the cumulative live birth rates obtained after cryopr eservation of either pronucleate (PN) zygotes or early-cleavage (EC) embryo s. Design: Prospective randomized study. Setting: University hospital. Patient(s): Three hundred eighty-two patients, involved in an IVF/ICSI prog ram from January 1993 to December 1995, who had their supernumerary embryos cryopreserved either at the PN (group I) or EC (group II) stage. For 89 pa tients, cryopreservation of EC embryos was canceled because of poor embryo development (group III). Frozen-thawed embryo transfers performed up to Dec ember 1998 were considered. Main Outcome Measure(s): Age, oocytes, zygotes, cryopreserved and transferr ed embryos, damage after thawing, cumulative embryo scores, implantation, a nd cumulative live birth rates. Result(s): The clinical pregnancy and live birth rates were similar in all groups after fresh embryo transfers. Significantly higher implantation (10. 5% vs. 5.9%) and pregnancy rates (19.5% vs. 10.9%; P less than or equal to .02 per transfer after cryopreserved embryo transfers were obtained in grou p I versus group II, leading to higher cumulative pregnancy (55.5% vs. 38.6 %; P less than or equal to .002 and live birth rates (46.9% vs. 27.7%; P le ss than or equal to .0001. Conclusion(s): The transfer of a maximum of three unselected embryos and fr eezing of all supernumerary PN zygotes can be safely done with significantl y higher cumulative pregnancy chances than cryopreserving at a later EC sta ge. (Fertil Steril(R) 2000;74:946-52. (C) 2000 by American Society for Repr oductive Medicine.)