In vitro fertilization (IVF) has evolved rapidly since its inception 40 years ago. Advancements include controlled ovarian stimulation with gonadotropin, which multiplied the number of oocytes retrieved, and embryo yopreservation,
which made it possible to conserve surplus embryos for further use, thus increasing cumulative live-birth rates after an IVF cycle. Observational studies1,2 and small randomized, controlled trials3-5 have shown higher pregnancy
rates and better perinatal outcomes with frozenembryo transfer than with fresh-embryo transfer. It has been hypothesized that frozen-embryo transfer may provide a more favorable intrauterine environment for embryo implantation and placentation by avoiding the supraphysiologic condition that occurs after ovarian stimulation.