Embryo freezing technologies have widely been used in human IVF practice and in animal industry. In this chapter, we will review the development of embryo freezing technology and the application of the method, which will concentrate on discussion of the arguments in favor of and against freezing, as well as the latest results of success rates, comparing them with the other basic assisted reproductive technologies methods. Then, we will present our viewpoint for the future application of embryo freezing methods and their place in reproductive medicine. The analysis of facts and suggestions should enable researchers to rethink the position of cryobiology in reproductive medicine. It should be considered that the method of cryopreservation is not only a technology for storing embryos but also a method of embryo treatment that can potentially improve the success rates in infertile couples. There is also a theory that describes the "treatment" effect of freezing an embryo, which may explain the higher success rates of frozen embryo transfer (FET) compared to fresh embryo transfer (ET). The authors of the "Theory about the Embryo-Cryo treatment" believe that freezing and thawing could activate endogenous survival and repair mechanisms in preimplantation embryos.

Background: With the accumulating evidence of semen difference between distinct populations we decided to conduct a population-based prospective study to assess the reproductive capabilities of the Bulgarian males. Objective: To evaluate the semen parameters in men from the general population of Bulgaria. To analyze the effects of BMI (Body mass index) and tobacco smoking on semen characteristics. Methods: 482 males without history of reproductive problems were included for the purpose of this study. The duration of our study was from April to May in 2016 and April in 2017. The volunteers were subdivided based on their smoking status and on their BMI. After semen collection, a conventional semen analysis was carried out manually by trained embryologists in an andrology laboratory according to WHO recommendations. The results were statistically analyzed and presented with their mean values. Result(s): After semen analysis we report a mean of 3.29 ml for semen volume, 40.68 × 10 6 /ml for spermatozoa concentration, 128.38 × 10 6 for total sperm count, 52.54% of total motile spermatozoa in the ejaculate and a mean percentage of morphologically normal sperm standing at 9.6%. We did not find a negative effect of tobacco smoking on semen quality; however higher BMI is associated with lower sperm concentration per ml. Discussion and Conclusion(s): This is the first contemporary study, assessing semen quality in Bulgarians. The results obtained here show the lower quality of semen in Bulgaria, compared to other countries. No statistical differences were found between the semen quality of nonsmokers and smokers. Higher BMI was found to be associated with lower sperm concentration per milliliter.