Neonatal outcomes following fresh as compared to frozen/thawed embryo transfer in in vitro fertilization

In vitro fertilization (IVF) and embryo cryopreservation have become increasingly common in recent years. As utilization increases, it is important to understand the clinical effects these technologies have on offspring, as well as the mechanisms behind these effects. Many epidemiologic studies have observed that pregnancies following IVF are more likely to be affected by obstetric complications such as pre-eclampsia, preterm birth, and small for gestational age neonates compared with naturally conceived pregnancies. There has been a great deal of research emerging suggesting that these differences are related to the supraphysiologic hormonal environment that results from ovarian superovulation. While pregnancies resulting from frozen embryo transfer are less likely to experience these complications, babies born after frozen transfer are more likely to be large for gestational age. Epigenetic studies point toward differential methylation of genes critical for growth that may be responsible for the increased incidence of larger neonates following transfer of vitrified embryos. Although it does appear that perinatal outcomes are improved by transferring frozen embryos instead of fresh, it would be premature at this time to recommend universal freeze-all protocols in all patients. This article, as part of the "IVF Reviews" special issue of this journal, aims to expound on the issues outlined above.