BACKGROUND Morulas with delayed growth sometimes coexist with blastocysts. There is still limited evidence regarding the optimal disposal of surplus morulas. With the advancement of vitrification, the freezing-thawing technique has been widely applied to zygotes with 2 pronuclei, as well as embryos at the cleavage and blastocyst stages. The freezing of morulas, however, has rarely been discussed. The purpose of this study was to investigate whether these poor-quality and slow-growing morulas are worthy of cryopreservation. METHODS This is a retrospective, observational, proof-of-concept study. A total of 1033 day 5/6 surplus morulas were cryopreserved from January 2015 to December 2018. The study included 167 women undergoing 180 frozen embryo transfer cycles. After the morulas underwent freezing-thawing procedures, their development was monitored for an additional day. The primary outcome was the blastocyst formation rate. Secondary outcomes were clinical pregnancy rate, live birth rate and abortion rate. RESULTS A total of 347 surplus morulas were thawed. All studied morulas showed delayed compaction (day 5, n = 329; day 6, n = 18) and were graded as having low (M1, n = 54), medium (M2, n = 138) or high (M3, n = 155) fragmentation. The post-thaw survival rate was 79.3%. After 1 day in extended culture, the blastocyst formation rate was 66.6%, and the top-quality blastocyst formation rate was 23.6%. The day 5 morulas graded as M1, M2, and M3 had blastocyst formation rates of 88.9, 74.0, and 52.8% (p < 0.001), respectively, and the top-quality blastocyst formation rates were 64.8, 25.2, and 9.0% (p < 0.001), respectively. The clinical pregnancy rate was 33.6%. CONCLUSIONS The post-thaw blastocyst formation rate was satisfactory, with approximately one-half of heavily fragmented morulas (M3) developing into blastocysts. Most of the poor-quality morulas were worth to freeze, with the reasonable goal of obtaining pregnancy and live birth. This alternative strategy may be a feasible approach for coping with poor-quality surplus morulas in non-PGS (preimplantation genetic screening) cycles.

中文翻译：

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冻融后延迟和/或不完全压实的多余桑的发育潜力。

背景技术发育迟缓的桑ula有时与胚泡共存。关于剩余桑的最佳处理方法的证据仍然有限。随着玻璃化的发展，冻融技术已广泛应用于具有2个原核的受精卵以及卵裂和胚泡期的胚胎。桑rarely的冻结，但是，很少讨论。这项研究的目的是调查这些劣质和生长缓慢的桑ula是否值得冷冻保存。方法这是一项回顾性，观察性，概念验证性研究。从2015年1月到2018年12月，总共冷冻保存了1033天的5/6天多余桑。该研究包括167名经历180个冷冻胚胎移植周期的妇女。桑ula经过冻融处理后，他们的发展受到了额外的一天的监控。主要结局是囊胚形成率。次要结果是临床妊娠率，活产率和流产率。结果总共解冻了347个桑surplus。所有研究的桑ula都表现出延迟压实（第5天，n = 329；第6天，n = 18），并且被分为低（M1，n = 54），中（M2，n = 138）或高（M3，n = 155）。解冻后存活率为79.3％。延长培养1天后，囊胚形成率为66.6％，优质囊胚形成率为23.6％。分级为M1，M2和M3的第5天桑的胚泡形成率分别为88.9、74.0和52.8％（p <0.001），最优质的胚泡形成率分别为64.8、25.2和9.0％（p <0.001）。临床妊娠率为33.6％。结论解冻后囊胚的形成率令人满意，大约有一半的重质桑（M3）发育为囊胚。大多数劣质桑mor都值得冻结，其合理目标是获得妊娠和活产。这种替代策略可能是在非PGS（植入前基因筛查）周期中应对劣质多余桑mor的可行方法。