**Acknowledgements**

It is well known that type of culture media (where are embryos after thawing) is very important for successful thawing process. This fact is often ignored and we believe that gentle appropriate

One of the new strategies is also "freeze all". In "freeze all cycle", all embryos are cryopreserved and later used after thawing in another reproductive cycle. This approach is very often used in cooperation with preimplantation genetic screening (PGS) of embryos before their transfer into the uterus. New trend in this approach is genetic screening of blastomere after biopsy at the fifth day of *in vitro* cultivation or later. There is no other way and all embryos must be cryopreserved and stored in liquid nitrogen. Embryos are thawed after final decision about

It was presented that implantation, clinical and ongoing pregnancy rates of ART cycles may be improved by performing cryoembryotransfer compared with fresh embryo transfer [31]. It can be explained by a better embryo endometrium synchrony achieved with endometrium preparation cycles. In frozen embryo transfers, endometrium priming may be achieved with the use of E2 and P, and the endometrial development can be controlled more precisely than

Frozen oocyte replacement is a technique where oocytes are retrieved, frozen, stored and fertilized only after thawing them for transfer. This technique helps women to preserve the future ability of having genetically related children at later point in life. It was first used for cancer patients before chemotherapy or radiotherapy. However, it can be also used for delaying motherhood for any reason, such as an absence of suitable partner or a work career. Large companies like Facebook or Apple have recently included social freezing for female employees as an employment benefit. Indeed, just as for fresh oocyte, the outcome of IVF with vitrified oocytes is highly dependent on maternal age. The most appropriate age for effective cryopre‐ servation is unknown, but ideally, it would be in the early to mid‐30s, before age at which woman's fertility naturally declines. Younger women have higher chance that they will never require these eggs. Elder women can be under risk of insufficient procedure with few amounts

Finally, it is well known that the embryologist training would have a major bearing on the vitrification outcome. Further vitrification procedural improvements using postvitrification chemical treatment would reduce the high sensitivity of oocytes and embryos to cryopreser‐ vation and provide valuable information during an advanced postcryopreservation thawing

of oocytes, aneuploidy oocytes and very low probability of pregnancy [33].

treatment after thawing can improve the survivability of oocytes and embryos.

embryo aneuploidy and their suitability for transfer into the uterus.

**6.2. New trends**

176 Cryopreservation in Eukaryotes

*6.2.1. Freeze all*

in cycles with gonadotropins [32].

*6.2.2. Social freezing*

**7. Conclusion**

Our work is supported by a grant MH CZ—DRO (FNBr, 65269705), and funds are received from the Faculty of Medicine, Masaryk University Brno, Czech Republic to junior researcher Michal Jeseta.
