Cryopreservation of zygotes and early cleaved human embryos.

Zygotes and 2- to 5-cell human embryos were frozen in 1,2-propanediol and sucrose; results of the first 50 cycles (45 patients) are presented. A total of 41 zygotes (17 attempts at thawing) were thawed, resulting in six singleton clinical pregnancies (15% per embryo; 35% per cycle), of which three delivered, one aborted, and two are ongoing. Fifty-seven cleaved embryos were thawed in 33 other cycles, resulting in four singleton and one twin pregnancy (11% per embryo; 15% per cycle), of which four delivered and one is ongoing. Depending on the cell stage, 61% to 81% of embryos survived cryostorage, but 2-cell embryos did not implant. One fifth of cryoinjury was due to the formation of cracks in the zona pellucida. The incidence of implantation was not enhanced when more than one freeze/thawed embryo was replaced, most pregnancies being obtained from single embryo replacements. At least 8% more births are expected in addition to conventional in vitro fertilization methods when the current policy of replacing three fresh embryos and freezing the remainder using this technique is applied. This method will result in two to four times more pregnancies per spare embryo, compared with other cryopreservation methods using older embryos.