Extended culture of vitrified-warmed embryos in day-3 embryo transfer cycles: a randomized controlled pilot study

Synchronization between embryonic stage and endometrium is vital to achieve a successful pregnancy. The objective of this study was to assess the implantation, clinical pregnancy and live birth rates of cryopreserved embryo transfer cycles using embryos after extended culture for 16h. A prospective randomized controlled pilot study was performed on women who underwent vitrified-warmed embryo transfer. Of the 540 women assessed for eligibility, 479 were randomly allocated to either extended culture for 16-18h (EC group, n=242) or conventional culture for 2h (control group, n=237). Endometrial preparation was the same in both groups. No significant differences were found between the extended culture and control groups respectively in clinical pregnancy rate per embryo transfer (42.48% versus 40.95%), implantation rate (21.79% versus 20.82%) or live birth rate per embryo transfer (37.61% versus 34.05%); however, the spontaneous reduction rate was lower in the extended culture group (10.04% versus 20.80%; P=0.032) In conclusion, extended culture of day-3 cleavage embryos for 16h would not influence the pregnancy outcome of day-3 cryopreserved embryo transfer cycles. Synchronization between embryonic stage and endometrium is vital to achieve a successful pregnancy and most implantation failures are possibly associated with inadequate endometrial receptivity or defects in the embryo-endometrium crosstalk. In this paper, we regulated synchronization via extended post-warming culture using conventional day-3 endometrial preparation in vitrified-warmed embryo transfer cycles and observed the clinical outcomes, including implantation, clinical pregnancy and live birth rates. Extended post-warming culture did not influence the clinical outcomes. This study implies that transfer of embryos at an advanced developmental stage may not affect implantation. This gives clinical doctors more flexibility in the performance of routine cryopreserved embryo transfer cycles.