الفهرس 
? CryopreservationOnly 14 pages are availabe for public view
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Abstract
Reproductive technologies, particularly in vitro fertilization (IVF), have provided great hope for infertile couples worldwide. Indeed, more than 5 million IVF babies have been born since the birth of the first, Louise Brown, in 1978 improvements in IVF had led to an increased number embryos not available for immediate transfer due to concern about multiple pregnancies. Cryopreservation of supernumerary embryos with subsequent frozen-thawed embryo transfer (FET) is an excellent solution to surplus embryos and has become a common practice in infertility centres. FET increases the pregnancy rate per oocyte retrieval and is a cost effective practice.
Successful FET cycle outcome depends on a delicate interaction between embryo quality and endometrial receptivity. So far, most studies conducted on FET cycle have focused on embryological factors such as blastomere survival and resumption of mitotic activity, while little attention has been paid to endometrial factors
The endometrium is normally a non-receptive environment for an embryo, except during the window. Implantation window is a self-limited period in which the endometriam has acquired the adequate morphologic and functional state for the blastocyst attachment.
In the implantation process an interaction between an embryo and the endometrium exists. This process seems to be affected by three crucial factors. These are embryo quality, endometrial receptivity and synchronization between the embryo developmental stage and endometrial profile during the window of implantation.
FET cycles need to be synchronized so that the age of the embryos after thawing corresponds to the age of the endometrium on the day of embryo transfer. The success of an FET program is closely linked to exact synchronization between endometrial maturation and embryo development. Such synchronization may be achieved in a natural cycle after spontaneous ovulation or after artificial preparation of the endometrium with exogenous steroids. Currently, the most popular protocol uses endometrial preparation with exogenous estrogens and progesterone after pituitary downregulation with a gonadotropin-releasing hormone (GnRH) analog so as to avoid spontaneous ovulation.