L26/P-219 High safety and effectiveness of “fast” dehydration/rehydration protocols in blastocyst cryopreservation: a consecutive cohort study of 1216 singleton live births

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Abstract Study question Do fast dehydration and rehydration protocols in blastocyst vitrification and warming provide comparable live birth rates (LBR), obstetrical and perinatal outcomes to traditional vitrification/warming protocols? Summary answer Fast dehydration/rehydration provide comparable LBR, pregnancy loss rate, birthweight, sex ratio, and incidences of maternal and infant medical complications compared to traditional vitrification/warming. What is known already While several large cohorts investigating one-step “fast” warming of blastocysts so far confirm equivalence to traditional “slow” protocols, relatively little data is available on “fast” vitrification. These new, simplified protocols, allow for improved operational efficiency and better workload management in clinics; however, a thorough analysis is required to ensure safety and effectiveness for both patient and offspring. Specifically, an investigation into the obstetric and perinatal outcomes of cycles including both “fast” vitrification and warming is urgently needed. Study design, size, duration This consecutive cohort study includes 3657 frozen single blastocyst transfers across 3 clinics (2023-2025). Protocols were aligned and consistent, stimulation to transfer, and were performed accordingly and in-line with patient aetiology. We modelled the relationship between “fast” versus “slow” protocols and outcomes with Generalized Additive Models, and linear and logistic regressions where appropriate. Two-tailed chi-square with Yates correction was used to examine pregnancy loss and obstetrical and perinatal outcomes; p < 0.05 was considered significant. Participants/materials, setting, methods All cycles were performed with autologous gametes, and without genetic testing. All transfers were SET. Collapsed blastocysts were vitrified on D5/D6 using a “slow” (S) (Sydney IVF, Cook Medical) or 2.5-minute “fast” (F) protocol (SAGETM, CooperSurgical). Warming was performed via a “slow” multi-step (S) or 1min 1.0M sucrose “fast” protocol (F) (SAGETM, CooperSurgical). Obstetric and perinatal data was requested from patients post due date, with subsequent calls to minimise loss to follow-up. Main results and the role of chance The overall LBR was 33.3% (1216/3657) with female age 34.8±4.5 (range:20.9–46.0). Four groups were established: traditional vitrification/multi-step warming (S/S, n = 688 LBR=34.0%), short vitrification/multi-step warming (F/S, n = 916 LBR=36.4%), traditional vitrification/one-step warming (S/F, n = 551 LBR=35.6%) and short vitrification/one-step warming (F/F, n = 1502 LBR=30.2%). S/S was set as the control group, and n = 7 (0.2%) cases were lost to follow-up. After controlling for maternal age, embryo quality, and day of vitrification, LBR in F/S, S/F and F/F were comparable to S/S (F/S:OR=1.16[0.92-1.46], p = 0.223; S/F:OR=1.02[0.77-1.34], p = 0.908; F/F:OR=0.91[0.73-1.13], p = 0.389). Interestingly, in women >35yrs (n = 1872), we identified a trend towards higher LBR in F/S (S/S LBR=26.5%; F/S LBR=30.4%, OR = 1.35[0.96-1.90], p = 0.081; S/F LBR=27.2%, OR = 1.04[0.68-1.57] p = 0.87; F/F LBR=21.0%, OR = 0.85[0.62-1.17], p = 0.32). A total of 157/1549 (10.1%) clinical pregnancies were lost before 7-weeks and a further 114 (7.4%) were lost later; both outcomes were comparable across groups (all p > 0.05) Once adjusted for gestation length and sex, birthweights were comparable across groups, as was the sex ration of female: male (p > 0.05). Complications were reported in 209/1549 (13.5%) of clinical pregnancies, comparable across groups. Neonatal issues were reported in 19/1216 (1.56%) cases, ranging from common observations like jaundice, hypospadias and tongue-tie to more serious conditions, and were also comparable across groups (p > 0.05). Limitations, reasons for caution Variables affecting the results may be unaccounted for by the study’s retrospective nature. Larger datasets may confirm whether a combination of reduced exposure to cryoprotectants combined with a gradual approach to warming (group F/S) may be beneficial to patients of advanced maternal age. Wider implications of the findings Overall, shorter, “faster” vitrification and warming protocols provide comparable LBR, obstetric and perinatal outcomes to traditional methods, providing further reassurance of their safety in addition to effectiveness. Substantial time savings associated with these protocols should improve operational efficiency and manage increasing workloads in clinical embryology laboratories. Trial registration number No

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