Extended Culture of Early Stage Embryos in Frozen-thawed Cycles

To investigate the impact of extended culture of early stage embryos on pregnancy outcome of frozen embryo transfer (FET). The survival rates of embryos after thawing and pregnancy outcome following FET were compared retrospectively between zygote and cleavage embryos which cultured to cleavage stage or extended cultured to blastocysts. A total of 425 zygote embryos in 67 cycles were thawed. After thawing, the survival rate was 94.4% and with an average transfer of 2.8 embryos, the clinical pregnancy rate was 55.2% (37/67). In 222 FET cycles, totally 1 270 cleavage stage embryos were thawed and the overall survival rates were 80.3%. With an average transfer of 2.7 embryos, the clinical pregnancy rate was 55.4% (123/222). A significantly lower percentage of degenerated embryos were found for zygotes (5.6%) than that for cleavage stage embryos (19.7%) (P<0.01). The clinical pregnancy rate was 53.4% (116/217) in the group of transfer at cleavage stage, while the clinical pregnancy rate was 61.1% (44/72) in the group of transfer at blastocyst stage (P>0.05). Although the clinical pregnancy rate was not different between patients with freeze-thaw zygote and cleavage stage embryo transfer, higher survival rate for zygote was shown compared with that for cleavage stage embryo. However, the present studies did not demonstrate that extended culture thawing embryos to blastocyst could achieve favor clinical outcome.