**1. Introduction**

The advent of embryo cryopreservation 25 years ago [1, 2] was revolutionary as well as critical in reproductive medicine. Cryopreservation and storage of gametes and embryos provide cost and procurement efficiencies in treatment options, which otherwise would be inaccessible without substantial financial resources.

Cryopreservation maximizes fertility potential per retrieval cycle, providing a repository for individual's gametes/embryos that may not exist elsewhere. Thus reproductive potential is not limited to reproductive years, but available as one manifests the need. Use of this technology has paved the way toward single embryo transfer (SET), thereby decreasing the risk of multiple gestation pregnancy and associated health risks.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Additional application has unfolded to address needs associated with convenience and transport for the purposes of using gestational carriers, family planning, travel, and using donor oocytes/embryos in support of non-fertile couples in conception and familial continuity.

Advances in cryopreservation are paralleled and even rooted in key developments in assisted reproductive technologies (ARTs). Historically, advanced culture techniques, complex culture media, and supplements specific to the support of late stage embryonic development attempted to mimic in vivo conditions [3]. Together with carefully controlled culture environment, blastulation rates have increased exponentially from decades prior [4, 5].

In addition to these supportive measures, newer technologies including time lapse morphological assessment allow for the development of observation-based algorithms as prognostic indicators of embryonic competence. Collectively these factors form the paradigm in increased opportunities for cryopreservation and subsequent improved selection criteria for single embryo transfer. In fact, this is the current trend.

The goal of healthy ART outcome should not be clouded by commercial success rates, and while not a mandate, single embryo transfer (SET) is now widely accepted as the default position in good prognosis patients. According to a meta-analysis of randomized controlled trials, SET versus double embryo transfer (DET) in a fresh In Vitro Fertilization (IVF) treatment cycle resulted in a lower pregnancy rate, lower rate of multiple births and preterm birth, and better odds of delivering a term singleton live birth. The reported SET versus DET pregnancy rate disparity is virtually eliminated with an additional frozen SET cycle [6]. However, the immediate consequence is that in order to achieve similar results, the patient may require/need multiple cycles of embryo transfers.

Approximately 30–50% of embryos make it to a blastocyst stage. The average number of embryos frozen per IVF cycle is age dependent: women of age >35 have fewer than two embryos frozen, while younger women responding better to ovarian stimulation and producing more eggs, result in a higher likelihood of having excess embryos available for freezing [7]. The Department of Health and Human Services estimates that in 2015 more than 600K frozen embryos were stored nationwide in the USA [8]. Figures for the year 2012 released by the Human Fertilization and Embryo Authority (UK) report that of the >3.5 million embryos created since 1991, 840K (24%) were cryopreserved for clinical use. In Canada, it is estimated that >60K frozen embryos are in storage [9]. The current trend of freezing all the embryos with no fresh embryo transfer [10] in IVF treatment would suggest these numbers will likely grow much faster. Despite this uncertainty, these values underscore the importance of cryopreservation technologies.
