THE MORPHOLOGICAL DYNAMICS OF BLASTOCYSTS VITRIFICATION IN EQUILIBRIUM SOLUTION: CLINICAL OUTCOMES AND INFLUENCING FACTORS.

During blastocyst vitrification, dehydration and permeation of the cryoprotectant cause the blastocyst to shrink and re-expand in the equilibrium solution. But some blastocysts do not re-expand. This study examines the clinical outcomes and influencing factors of embryos without re-expansion. This study is based on data collected on 9083 vitrified/warmed transferred blastocysts between May 2013 to August 2020. Blastocysts that did not re-expand to more than 80% of their former volume in equilibrium solution (15 min) were defined as the NON group (n=470). The clinical results were compared with the blastocysts which re-expanded (RE group; n=8613). The clinical outcomes were evaluated in terms of followings: ① patient characteristics (under 38 years old or 38 years old and over), ②embryo quality (good or poor), ③ the day of blastocyst culture (D5 or D6). In addition, we evaluated which factors influenced those not re-expanded, ④ culture medium (G series or Global) and ⑤ fertilization methods (conventional IVF or ICSI). The pregnancy rates were significantly lower in the NO group (① patient characteristics: under 38 years old, 48.6% vs. 58.0%, over 38 years old and over, 22.5% vs. 38.8%; ② embryo quality: good 43.2% vs. 55.1%, poor 25.2% vs. 35.5%; ③ the day of blastocyst culture: D5 37.6% vs. 51.2%, D6 20.2% vs. 31.4%; p<0.01, respectively). Abortion rates did not differ between the two groups. We compared no re-expansion rates under 5 conditions and obtained results from ① 4.3% vs. 5.9% (under 38 years old, 38 years old and over), ② 3.4% vs. 7.9% (good, poor), ③ 4.8% vs. 7.9% (D5, D6), ④ 7.3% vs. 4.7% (G series, Global), ⑤ 4.9% vs. 5.6% (conventional IVF, ICSI). There were significant differences in maternal age, embryo quality, day of blastocyst culture, and culture medium (p<0.01). During vitrification, the absence of re-expansion has a negative impact on pregnancy rates. This suggests that embryologists should carefully observe the presence or absence of re-expansion in the equilibrium solution during the vitrification, and that it is desirable to preferentially select embryos showing re-expansion in frozen embryo transfer. Further investigation is necessary to search for influencing factors of no re-expansion.