Comparative neonatal outcomes of vitrified versus fresh embryo transfers: A systematic review and meta-analysis

Objective: Vitrification prevents ice formation within the high concentration of cryoprotectant agents and allows the cells and extracellular milieu to solidify into a glass-like state. This study aimed to provide objective information on the available data regarding singleton births comparing embryo vitrification with fresh embryo transfers. Methods: PubMed and Cochrane databases were searched for studies conducted up to 2017. Low birth weight (LBW), very low birth weight (VLBW), preterm birth (PTB), very PTB, small for gestational age (SGA), large for gestational age, perinatal mortality, cesarean section, and preeclampsia were measured. Results: The search returned 220 articles, and 13 of these met the inclusion criteria. A significantly lower risk of SGA birth was observed in the vitrified embryo transfer group than in the fresh transfer group. Conversely, a higher risk of very PTB, VLBW, large size for gestational age, and perinatal mortality was observed more frequently in the vitrified embryo transfer group. Conclusions: Vitrified embryo transfer was associated with the improved outcome of neonates regarding the SGA rate compared with fresh embryo transfer, but included a higher risk of very PTB, VLBW, large size for gestational age, and perinatal mortality. Caution should be exercised in interpreting these findings given the low level of evidence of the studies.