Clinical outcomes following cryopreservation of blastocysts by vitrification or slow freezing: a population-based cohort study

STUDY QUESTION: What are the clinical efficacy and perinatal outcomes following transfer of vitrified blastocysts compared with transfer of fresh or of slow frozen blastocysts? SUMMARY ANSWER: Compared with slow frozen blastocysts, vitrified blastocysts resulted in significantly higher clinical pregnancy and live delivery rates with similar perinatal outcomes at population level. WHAT IS KNOWN ALREADY: Although vitrification has been reported to be associated with significantly increased post-thaw survival rates compared with slow freezing, there has been a lack of general consensus over which method of cryopreservation (vitrification versus slow freezing) is most appropriate for blastocysts. STUDY DESIGN, SIZE, DURATION: A population-based cohort of autologous fresh and initiated thaw cycles (a cycle where embryos were thawed with intention to transfer) performed between January 2009 and December 2011 in Australia and New Zealand was evaluated retrospectively. A total of 46 890 fresh blastocyst transfer cycles, 12 852 initiated slow frozen blastocyst thaw cycles and 20 887 initiated vitrified blastocyst warming cycles were included in the data analysis. PARTICIPANTS/MATERIALS, SETTING, METHODS: Pairwise comparisons were made between the vitrified blastocyst group and slow frozen or fresh blastocyst group. A Chi-square test was used for categorical variables and t-test was used for continuous variables. Cox regression was used to examine the pregnancy outcomes (clinical pregnancy rate, miscarriage rate and live delivery rate) and perinatal outcomes (preterm delivery, low birthweight births, small for gestational age (SGA) births, large for gestational age (LGA) births and perinatal mortality) following transfer of fresh, slow frozen and vitrified blastocysts. MAIN RESULTS AND THE ROLE OF CHANCE: The 46 890 fresh blastocyst transfers, 11 644 slow frozen blastocyst transfers and 19 978 vitrified blastocyst transfers resulted in 16 845, 2766 and 6537 clinical pregnancies, which led to 13 049, 2065 and 4955 live deliveries, respectively. Compared with slow frozen blastocyst transfer cycles, vitrified blastocyst transfer cycles resulted in a significantly higher clinical pregnancy rate (adjusted relative risk (ARR): 1.47, 95% confidence intervals (CI): 1.39-1.55) and live delivery rate (ARR: 1.41, 95% CI: 1.34-1.49). Compared with singletons born after transfer of fresh blastocysts, singletons born after transfer of vitrified blastocysts were at 14% less risk of being born preterm (ARR: 0.86, 95% CI: 0.77-0.96), 33% less risk of being low birthweight (ARR: 0.67, 95% CI: 0.58-0.78) and 40% less risk of being SGA (ARR: 0.60, 95% CI: 0.53-0.68). LIMITATIONS, REASONS FOR CAUTION: A limitation of this population-based study is the lack of information available on clinic-specific cryopreservation protocols and processes for slow freezing-thaw and vitrification-warm of blastocysts and the potential impact on outcomes. WIDER IMPLICATIONS OF THE FINDINGS: This study presents population-based evidence on clinical efficacy and perinatal outcomes associated with transfer of fresh, slow frozen and vitrified blastocysts. Vitrified blastocyst transfer resulted in significantly higher clinical pregnancy and live delivery rates with similar perinatal outcomes compared with slow frozen blastocyst transfer. Comparably better perinatal outcomes were reported for singletons born after transfer of vitrified blastocysts than singletons born after transfer of fresh blastocysts. Elective vitrification could be considered as an alternative embryo transfer strategy to achieve better perinatal outcomes following Assisted Reproduction Technology (ART) treatment. STUDY FUNDING/COMPETING INTERESTS: No specific funding was obtained. The authors have no conflicts of interest to declare.