Background: Controlled ovarian stimulation (COS) has a negative effect on the endometrial receptivity compared with natural menstrual cycle. Whether it's necessary to postpone the first frozen embryo transfer (FET) following a freeze-all strategy in order to avoid any residual effect on endometrial receptivity consequent to COS was inconclusive. Objective: The purpose of this retrospective study was to explore whether the delayed FET improve the live birth rate and neonatal outcomes stratified by COS protocols after a freeze-all strategy. Methods: A total of 4404 patients who underwent the first FET cycle were enrolled in this study between April 2014 to December 2017, and were divided into immediate (within the first menstrual cycle following withdrawal bleeding) or delayed FET (waiting for at least one menstrual cycle and the transferred embryos were cryopreserved for less than 6 months). Furthermore, each group was further divided into two subgroups according to COS protocols, and the pregnancy and neonatal outcomes were analyzed between the immediate and delayed FET following the same COS protocol. Results: When FET cycles following the same COS protocol, there was no significant difference regarding the rates of live birth, implantation, clinical pregnancy, multiple pregnancy, early miscarriage, premature birth and stillbirth between immediate and delayed FET groups. Similarly, no significant differences were found for the mean gestational age, the mean birth weight, and rates of low birth weight and very low birth weight between the immediate and delayed FET groups. The sex ratio (male/female) and the congenital anomalies rate also did not differ significantly between the two FET groups stratified by COS protocols. Conclusion: Regardless of COS protocols, FET could be performed immediately after a freeze-all strategy for delaying FET failed to improve reproductive and neonatal outcomes.

Background Multiple pregnancies are associated with significant complications and health risks for both mothers and infants. Single blastocyst transfer (SBT) is a logical and effective measure to reduce the incidence of multiple pregnancy with assisted reproductive technology (ART). Whether it is suitable for everyone undergoing SBT was inconclusive, in view of the consideration of embryo quality and patients’ age. Therefore, this study aimed to explore live birth rate (LBR) and neonatal outcomes of different quantities and qualities of blastocysts in patients stratified by age, using a cutoff of 35 years, who required whole embryo freezing and underwent a subsequent frozen thawed transfer (FET) cycle. Methods Atotal of 3,362 patients were divided into five groups: group A (n=1569) received a single good-quality blastocyst, group B (n=1113) received two good-quality blastocysts, group C (n=313) received one good-and one average-quality blastocyst, group D (n=222) received two average-quality blastocysts, and group E (n=145) received one average-quality blastocyst. Results For patients who received good-quality blastocysts, irrespective of age, the LBR of double blastocyst transfer (DBT) was about 50–65% and the multiple pregnancy rate (MPR) was 40–60%; however, the LBR of SBT was 40–55%, and the MPR was 3.5–6.3%. For patients who only had average-quality blastocysts, the MPR of double average-quality blastocyst transfer was as high as 30–50%. Moreover, about 70–90% of preterm births resulted from multiple pregnancies, and about 85–95% of low birth weight babies come from multiple pregnancies. The neonatal outcomes (gestational age, birth weight, and birth height) of DBT were significantly lower than those of SBT regardless of age, and this statistical difference disappeared if the patients were subgrouped by singleton or twin. There is no significant difference in neonatal outcomes between single good-quality blastocyst and single average-quality blastocyst transfer. Conclusions SBT is a preferable option for patients regardless of age when good-quality blastocysts are available. For patients who only had average-quality blastocysts, they should be informed that DBT was associated with higher multiple pregnancy and adverse neonatal outcomes when compared with SBT regardless of age, suggesting that the practice of SBT is also feasible for these patients.