Molecular and Cellular Endocrinology 169 (2000) 55 – 62 Frozen pronuclear oocytes: advantages for the patient N. Nikolettos a, *, S. Al-Hasani b a Democritus Uniersity of Thrace, Faculty of Medicine, Alexandroupolis, Greece b Department of Obstetrics /Gynecology, Medical Uniersity Lu ¨beck, Ratzeburger Allee 160, D-23538 Lubeck, Germany Abstract Since the first reported pregnancy in a human being after a frozen/thawed eight cell stage preembryo, cryopreservation of preembryos has been integrated as an important element of assisted reproductive technologies (ART). The cryopreservation technique has brought several advantages to ART. It allows the transfer of a limited number of embryos in the collection cycle, thereby reducing the risk of multiple pregnancies, and the patients have a reservoir of excess embryos for additional transfers. This maximises the number of embryo transfers per oocyte retrieval, while at the same time increasing the cumulative pregnancy rate from a given treatment cycle. Also, the ability to freeze all the embryos obtained and transfer at a subsequent cycle is useful in the avoidance of hyperstimulation syndrome, or when factors that may jeopardize implantation are apparent. Freezing of oocytes in a pronuclear stage has a valuable role in the management of infertility. Supernumerary zygotes can be cryopreserved safely for future transfer, avoiding additional inconvenience for the patients. The freezing/thawing technique does not have any adverse effects on oocytes fertilized microsurgically. Pronuclear stage oocytes eventually survive the cryopreservation procedure better, yielding after culture cleaved embryos appropriate for transfer, which could increase the implantation rate. We believe that the cryopreservation of cleaved embryos, which is problematic, can be safely replaced by this procedure. This is not only an advantage for society as a whole, but also for the people involved in the process, as there should be no ethical or moral conflict for the patients or for the laboratory staff about discarding this material. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Cryopreservation; Oocytes; Frozen – thawed www.elsevier.com/locate/mce 1. Introduction Today cryopreservation of preembryos has become an integrated part of assisted reproductive technologies (ART) since the first human pregnancy from frozen – thawed embryo reported by Trounson and Mohr (Trounson and Mohr, 1983) and the first birth by Zeilmaker et al. (Zeilmaker et al., 1984). Cryopreserva- tion of embryos has brought several advantages in ART. It allows to transfer a limited number of embryos in the collection cycle, thereby, reducing the chance of a high range of multiple pregnancy and the patients have a reservoir of embryos for additional transfers, maximizing the number of embryo transfers per oocyte retrieval and increasing the cumulative pregnancy rate from a given treatment cycle. Also, freezing all the embryos obtained and transferring at a subsequent cycle it may be decided to avoid hyperstimulation syn- drome or when factors which may jeopardize implanta- tion are apparent. Human embryos can be cryopreserved and thawed at a range of developmental stages using various freezing methods, with varying degrees of success (Trounson and Mohr, 1983; Cohen et al., 1985; Al-Hasani et al., 1991). They commonly utilize cryoprotectants such as dimethylsulphoxide (DMSO), glycerol, or 1,2 propane- diol (PROH) (Lassalle et al., 1985; Veeck et al., 1993; Van der Elst et al., 1995; Al-Hasani et al., 1996). Programmable, controlled-rate freezing equipment have been employed to provide slow rates of cooling before immersion and storage into liquid nitrogen (Ashwood- Smith, 1986; Troup et al., 1990). Some success has also been reported using ultrarapid freezing as a possible alternative to conventional slow-cooling methods (Trounson et al., 1987; Gordts et al., 1990). The largest percentage of live births in the literature have been reported with cryopreservation of supernumerary em- bryos at pronuclear stage. The pregnancy rate after transfer of cryopreserved embryos is low compared to fresh embryos (American Society for Reproductive Medicine, 1995, 1996). This * Corresponding author. 0303-7207/00/$ - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0303-7207(00)00352-X