ABSTRACT
Controlled ovarian hyperstimulation (COH) is the key factor for the success of in vitro fertilization-embryo transfer (IVF-ET) cycles. Its aim is the recruitment of multiple, fertilizable, healthy oocytes with the avoidance of severe ovarian hyperstimulation syndrome (OHSS). Only 1 to 3 of the embryos obtained are transferred into the uterus in each cycle, in order to reduce the risk of multiple gestation; the remainding are cryopreserved for future replacement. Cryopreservation of extra embryos, thus, provides further possibilities for conception after the initial fresh transfer, leading to a higher cumulative pregnancy rate per cycle. Cryopreservation of all embryos may be also helpful in patients at high risk of late severe OHSS1 and in patients in whom endometrial pathology (endometrial polyp, poor endometrial development, etc. is detected during COH. In these situations, transfer of the cryopreserved embryos can be conducted in a subsequent non-stimulated cycle or after the intrauterine abnormality is treated, respectively. Embryo cryopre-servation may also simplify donor oocyte programs by obviating the need for meticulous synchronization between donor and recipient.2
