Introduction: Evidence-based in vitro fertilization treatment of fresh versus frozen embryo transfer: peeling away the layers of the onion.

This introduction to this Views and Reviews examines the current evidence for elective fresh versus frozen embryo transfer, delineates complexities that arise as we sift through the available evidence, and speculates on the ideal design of studies to arrive at an evidence-based consensus. Current gaps in the literature, including a lack of reporting on important outcomes such as live birth and maternal and perinatal adverse events, are noted. The difficulty in designing trials is reviewed, including the choice of the time point in the cycle to randomize patients, how to account for all the patients and embyros that are involved in the trial, whether to incorporate prenatal genetic screening, and the best type of study design to address the risk/benefit ratio of these practices.