Embryonic development and pregnancy from fresh and cryopreserved sibling pronucleate human zygotes

The number of pronucleate (PN) zygotes that cleave, rates of cell division, and pregnancy rates were compared from fresh and frozen sibling PN zygotes. Of 57 patients with randomly selected zygotes frozen at the PN stage, 26 have had 82 PN zygotes thawed with 76 (92.7%) morphologically intact immediately post-thaw. There was no significant difference (P > 0.05) in the proportion of fresh (110/123 [89.4%]) and frozen-thawed intact (62/76 [81.6%]) sibling PN zygotes that had cleaved by transfer. The number of fresh versus frozen-thawed sibling PN zygotes that developed into 2-, 3-, 4-, 5-, 6-, 7-, and 8-cell embryos after 24 hours in culture were not significantly different. The numbers of pregnancies per patient cycle for fresh and frozen-thawed sibling PN zygotes were 12/60 (20.0%) and 5/27 (18.5%), respectively (P > 0.05). The current cumulative pregnancy rate per retrieval in all cycles with frozen zygotes is 28.3%, considerably higher than observed in single transfers of embryos without cryopreservation (19.8%); predicted cumulative pregnancy rate after eventual transfer of all frozen zygotes is 37.9%. It is concluded that embryo survival and pregnancy rates of cryopreserved PN zygotes are very similar to those from their fresh sibling counterparts and cryopreservation substantially enhances in vitro fertilization pregnancy attainment.