Effect of developmental stage of embryo at freezing on pregnancy outcome of frozen-thawed embryo transfer.

Background: The study aim was to investigate the impact of the developmental stage of embryos on pregnancy outcome of frozen embryo transfer (FET).

Methods: The survival rates of embryos after thawing and pregnancy outcome following FET were compared retrospectively between three cryopreservation strategies utilizing either zygote, day 2 or day 3 embryo freezing.

Results: A total of 4006 embryos was analysed in 1657 thaw cycles.