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(Abstracted from N Engl J Med 2018;378:137–147)After its introduction in 1978, in vitro fertilization (IVF) was traditionally performed by the transfer of fresh embryos. With improvements in IVF technology, the number of multiple pregnancies increased, leading to a gradual reduction in the number of embryos transferred. To allow subsequent transfer when the fresh cycle was unsuccessful, the technique of embryo freezing was developed. A recent large, randomized trial comparing frozen-embryo transfer with fresh-embryo transfer in women with polycystic ovary syndrome (PCOS) who were undergoing IVF showed that the live-birth rate was significantly higher with frozen-embryo transfer. Whether frozen-embryo transfer also results in higher live-birth rates in women with infertility not associated with PCOS is unknown.