STUDY QUESTION What are the differences in ease of use between two different embryo transfer (ET) techniques: the preload direct approach and the afterload approach. SUMMARY ANSWER The afterload technique seems to reduce the rate of difficult ETs. WHAT IS KNOWN ALREADY Numerous published trials now document that the ET procedure has an impact on pregnancy and delivery rates after IVF. Difficult transfers should be avoided, as they reduce implantation and pregnancy rates. Preload direct ETs with soft catheters under ultrasound guidance is currently considered the best procedure. However, when using soft catheters, it is not known which technique is preferable or which one should be implemented to reduce the operator factor. STUDY DESIGN, SIZE, DURATION This prospective randomised unblinded controlled clinical trial, included 352 ultrasound-guided ETs assigned to either direct ET or afterload ET, between September 2017 and March 2019. The sample size was calculated based on the historical rate of difficult ETs encountered between 2014 and 2015 with a direct ET procedure. PARTICIPANTS/MATERIALS, SETTING, METHODS The inclusion criteria were women 18–38 years old, with BMI between 18 and 28, receiving a single-thawed blastocyst transfer. The exclusion criteria were use of testicular sperm and preimplantation genetic testing (PGT) cycles. The primary outcome was the rate of difficult or suboptimal transfers defined as: advancement of the outer sheath (specific for the direct transfer), multiple attempts, use of force, required manipulation, use of a stylet or tenaculum, dilatation, or use of a different catheter. The secondary outcome was clinical pregnancy rate. MAIN RESULTS AND THE ROLE OF CHANCE A total of 352 frozen ETs were randomised, with 176 patients in each group. The two arms were homogeneous for female and male age, female BMI, duration of infertility, secondary infertility, previous deliveries or miscarriages, myomas, previous surgery to the uterine cavity, cycle day at ovulation trigger, freeze all cycles, first transfers, indication for treatment, endometrial preparation protocol and duration, endometrial thickness, and blastocyst grade at vitrification. Across the entire population, 85 (24.1%) ETs were defined as difficult. The rate of difficult transfers was significantly higher in the direct ET group than in the afterload group: 68 (38.6%) versus 17 (9.7%), respectively (OR 0.17, 95% CI 0.09–0.30, P < 0.001). The mean percentage in the rate of difficult transfers per operator was 22.5% (SD ± 14.5%), of which 36.1% (SD ± 23.4%) were in the direct group compared with 8.6% (± 8.2%) in the afterload group (P < 0.001). The difficult transfer rate among operators varied from 0 to 43.8% (0–77.8% in the direct group and 0 to 25.0% in the afterload group). The clinical pregnancy rates (42.0% vs 48.3%, P = 0.239 in the direct and afterload groups, respectively) were not significantly different between the groups. LIMITATIONS, REASONS FOR CAUTION There were 18 experienced operators who participated in the trial. Conclusions about the pregnancy rate should not be generalised, since the sample analysis was not performed on this outcome and, although clinically relevant, the difference was not significantly different. WIDER IMPLICATIONS OF THE FINDINGS The rate of difficult transfers was significantly higher in the direct ET group compared with the afterload ET group, although a wide variation was observed among operators. Further studies regarding the association between transfer technique and ART outcomes are required. STUDY FUNDING/COMPETING INTEREST(S) No specific funding was sought and there are no competing interests. TRIAL REGISTRATION NUMBER NCT03161119. TRIAL REGISTRATION DATE 5 April 2017. DATE OF FIRST PATIENT'S ENROLMENT 26 September 2017.

中文翻译：

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更进一步：比较胚胎移植的直接和后加载技术的随机单中心试验

研究问题 两种不同的胚胎移植 (ET) 技术在易用性方面有何不同：预加载直接方法和后加载方法。总结答案 后负荷技术似乎降低了困难 ET 的发生率。已知情况 许多已发表的试验现在证明 ET 程序对 IVF 后的怀孕和分娩率有影响。应避免困难的转移，因为它们会降低植入率和妊娠率。在超声引导下用软导管预加载直接 ET 被认为是目前最好的程序。然而，当使用软导管时，不知道哪种技术更可取或应该实施哪种技术以减少操作者因素。研究设计、规模、持续时间这项前瞻性随机非盲对照临床试验，包括 2017 年 9 月至 2019 年 3 月期间分配给直接 ET 或后负荷 ET 的 352 名超声引导 ET。样本量是根据 2014 年至 2015 年期间通过直接 ET 程序遇到的困难 ET 的历史率计算的。参与者/材料、设置、方法 纳入标准为 18-38 岁的女性，BMI 在 18-28 之间，接受单次解冻的囊胚移植。排除标准是使用睾丸精子和植入前基因检测（PGT）周期。主要结果是困难或次优转移的发生率，定义为：外鞘的推进（特定于直接转移）、多次尝试、使用武力、需要的操作、使用管心针或钩针、扩张或使用不同的导管。次要结果是临床妊娠率。主要结果和机会的作用 共有 352 名冷冻 ET 被随机分组​​，每组 176 名患者。两组的女性和男性年龄、女性 BMI、不孕持续时间、继发性不孕、以前的分娩或流产、肌瘤、以前的子宫腔手术、排卵触发的周期天、冻结所有周期、第一次转移、适应症治疗、子宫内膜准备方案和持续时间、子宫内膜厚度和玻璃化时的囊胚等级。在整个人群中，85 (24.1%) 名 ET 被定义为困难。直接 ET 组的困难转移率显着高于后负荷组：分别为 68 (38.6%) 和 17 (9.7%) (OR 0.17, 95% CI 0.09–0.30, P < 0.001)。每位操作者的困难转移率平均百分比为 22.5% (SD ± 14.5%)，其中直接组为 36.1% (SD ± 23.4%)，后负荷组为 8.6% (± 8.2%)。 P＜0.001)。操作员之间的困难转移率从0%到43.8%不等（直接组为0%~77.8%，后载组为0%~25.0%）。临床妊娠率（直接负荷组和后负荷组分别为 42.0% 和 48.3%，P = 0.239）在各组之间没有显着差异。限制、谨慎的原因 有 18 名经验丰富的操作员参与了试验。关于妊娠率的结论不应一概而论，因为没有对这一结果进行样本分析，尽管临床相关，但差异没有显着差异。研究结果的更广泛意义 尽管在操作者之间观察到差异很大，但直接 ET 组的困难转移率明显高于后负荷 ET 组。需要进一步研究转移技术与 ART 结果之间的关联。研究资金/竞争利益 没有寻求具体资金，也没有竞争利益。试用注册号 NCT03161119。试用注册日期 2017 年 4 月 5 日。第一位患者注册日期 2017 年 9 月 26 日。研究资金/竞争利益 没有寻求具体资金，也没有竞争利益。试用注册号 NCT03161119。试用注册日期 2017 年 4 月 5 日。第一位患者注册日期 2017 年 9 月 26 日。研究资金/竞争利益 没有寻求具体资金，也没有竞争利益。试用注册号 NCT03161119。试用注册日期 2017 年 4 月 5 日。第一位患者注册日期 2017 年 9 月 26 日。