Background:Previous studies have shown that in young women, single blastocyst transfer can achieve satisfactory pregnancy results, but there are few studies on the difference between fresh and frozen-thawed single blastocyst transfer. In this retrospective analysis, the purpose of this study was to compare the pregnancy outcomes of young patients who received fresh or frozen-thawed single blastocyst transfer, so as to provide further clinical strategies for single blastocyst transfer.Methods: A

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... retrospective analysis of patients aged ≤35 years old undergoing single blastocyst transfer was performed from January 2018 to December 2018 in the reproductive center of the Second Affiliated Hospital of Wenzhou Medical University. A total of 901 patients were involved and were divided into two groups according to the type of transfer cycle, 693 of which were in the fresh embryo transfer cycle (group A) and 208 of which were in the frozen-thawed embryo transfer cycle (group B). The laboratory and pregnancy outcomes were compared between the groups. Results:The number of oocytes retrieved in group B was significantly more than in group A ( P<0.05). The early miscarriage rate in group B was higher than in group A, with significant difference(P<0.05). There were no significant differences in other basic conditions and obstetric pregnancy outcomes between the two groups.Conclusions:Favorable pregnancy outcomes could be obtained in both fresh and frozen-thawed single blastocyst transfer in young patients without increasing the birth defect rate. For patients with risk factors of OHSS such as more oocyte retrieval, ovarian hyper-response, or risk factors of ectopic pregnancy such tubal infertility and the history of ectopic pregnancy, frozen-thawed single blastocyst transfer could be highly recommended in clinical practice. However, since the early miscarriage rate is higher in frozen-thawed embryo transfer group, further research is needed to figure out the cause and possible solutions.