P-116 Relationship between exposure in equilibration solution pre-vitrification for human collapsed blastocysts and miscarriage rate in ART treatments

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Abstract Study question To explore the relationship between equilibration time pre-vitrification of human collapse blastocyst and clinical pregnancy and miscarriage rate in patients undergoing ART treatments Summary answer The miscarriage rate was significantly lower in the group with a short equilibration time of 7-8 minutes compared to the group of 9-10 minutes What is known already The introduction of vitrification represents one of the most important advancement in ART. However, protocols currently applied to cryopreserve human embryos still have some weak points that might be improved. A critical aspect is represented by the high concentration of cryoprotectants (CPAs) used during the vitrification and some of this CPAs might impact negatively cellular metabolism and function. Thus, this study investigated whether a shorter time in the equilibration solution before vitrification of artificially collapsed blastocysts might have an impact on survival and pregnancy outcomes, as well as live birth rate and risk of miscarriage in patients undergoing ART. Study design, size, duration This prospective study was performed at the Centre for Reproductive Medicine, Haikou Mary Hospital, China from March 2018 to May 2022. Informed consent for experimentation with human subjects was obtained before the patients start the ovarian stimulation. Female age <35 years, and causes of infertility included male factors, female infertility, and unexplained infertility. Following the fresh embryo transfer, supernumerary good quality blastocysts ≥ 2, according to Gardner’s score were vitrified and allocated as discussed below Participants/materials, setting, methods The study included a total of 831 expanded blastocysts, which were divided into two groups according to the equilibration time before vitrification: group (A) 7-8 minutes (413); and group (B) 9-10 minutes (418). Expanded blastocysts (grade 3 or more) were artificially shrunk by applying one or two laser pulses. Patients were included in the study only when at least two blastocysts were available for vitrification, in order to be allocated one in each group Main results and the role of chance Results: A total of 831 vitrified-warmed blastocysts were analysed in this study, of which 825 survived at the warming step (99.3%: 825/831). All surviving blastocysts were transferred in 585 embryo transfers. Overall, the clinical pregnancy rate per transfer was CPR: 68.5% (401/585), and live birth and miscarriage rates: 61.0% (357/585) and 11.0% (44/401). No significant differences were observed between the two groups regarding the mean age of patients, the average number of blastocysts transferred, the basal FSH, BMI, and infertility duration. Results show the same survival rate after warming for group A (99.3%) and group B (99.3%), as well as similar CPR (A: 69.1% versus B: 68.0%).The live birth (A: 63.8% versus B: 58.3%) and multiple gestation rates (A: 20.8% versus B: 23.5%) were comparable in the two groups. When analysing the overall miscarriage rate, data displayed a statistically significant difference (P < 0.05) in favour of group A (7.6%) compared to group B (14.2%). There were no differences between the two groups concerning the prevalence of male babies (A: 57.1% and B: 55.4%), average gestational length (A: 38.67±1.37 versus B: 38.33±1.21), preterm birth rate (A: 19.2% versus B: 20.6%), and birth weight (A: 2.95±0.58 versus B: 3.05±0.63 kg). Limitations, reasons for caution This is not a randomized controlled trial, it is a prospective observational cohort study aiming to calculate the effect of a shorted ES time on the risk of miscarriage. Also, potential confounding factors due to the heterogeneous nature of the sample investigated may impair the validity of our conclusions. Wider implications of the findings Results demonstrate that a shorter equilibration time resulted in optimal survival, clinical pregnancy, and live birth rates compared to exposure to ES for 9-10. Thus, suggesting that a longer exposure to the ES is not needed, and might negatively effect cellular metabolism and function, increasing the risk of miscarriage. Trial registration number Not applicatione