ABSTRACT

Traditional in vitro fertilization (IVF), in which fresh embryos are transferred to the patient’s uterus following controlled ovarian stimulation (COS), is limited by a temporally inflexible relationship between embryonic and uterine development. COS results in supraphysiologic hormone levels that may accelerate uterine histological development and impair uterine receptivity by inducing embryo–endometrium asynchrony, particularly when embryos develop slowly. Additionally, genetic analyses of embryos sometimes require a delay that might preclude fresh transfer, necessitating cryopreservation. Furthermore, fresh autologous embryo transfer is associated with increased risk of ovarian hyperstimulation syndrome (OHSS) and also with certain increased perinatal risks. For these reasons, it is becoming increasingly common to “segment” the IVF cycle by temporally separating the transfer from the retrieval through embryo or oocyte cryopreservation.