Factors affecting survival and implantation of cryopreserved human embryos.

Five- to ten-cell embryos and expanded blastocysts from 68 patients were thawed in an attempt to establish pregnancy. Three fresh embryos had been replaced unsuccessfully in these patients. Forty-five patients had intact freeze-thawed embryos replaced and 12 became clinically pregnant. Ten of these pregnancies have now advanced beyond week 24. The preliminary results presented here demonstrate that significantly more expanded blastocysts survive cryopreservation than cleaving embryos. Faster-growing embryos did not survive better than slowly growing embryos, but the incidence of implantation was higher with faster-developing embryos. Significantly more blastocysts with a "normal" morphology survived cryostorage than those scored as "irregular" (77 versus 7%). A similar trend was observed when normal and irregular cleaving embryos were frozen, but the difference was not significant. The severity of contraction of blastocysts upon the addition of cryoprotectant increased the incidence of survival. The proportion of patients whose embryos had holes in the zona pellucida following cryopreservation was significantly higher when embryos were frozen at the cleaving stage (39%) rather than the expanded blastocyst stage (16%). Almost all embryos with damaged zonae degenerated. The proportion of cleaving embryos that survived cryostorage was inversely correlated with the number of follicles aspirated.