LAUSR

We read the article entitled, “Effect of the time for embryo transfer from oocyte retrieval on clinical outcomes in freezeall cycles: a retrospective cohort study” by Chen et al. [1], which was published in the January 2019 issue of Archives of Gynecology and Obstetrics, with great interest. This study concluded that there was no significant difference between the first frozen embryo transfer (FET) performed in the first menstrual cycle group and the FET performed in the subsequent menstrual cycle group in terms of implantation, pregnancy, and live birth rates. The article pointed out that endometrial thickness is the deciding factor for higher pregnancy rate. On this basis, it was believed that the first FET could be performed as long as the endometrium reached a certain thickness, rather than being delayed until the subsequent menstrual cycle. The initiation time of FET after oocyte retrieval has always been a problem that plagues doctors engaged in assisted reproductive technology [2]. Most reproductive centers usually initiate FET during the third menstrual cycle in patients after oocyte retrieval. For patients who do not need pretreatment before FET, such as hydrosalpinx and thin endometrium, the waiting period is indeed too long. In addition, patients often express a strong desire for transplantation as soon as possible. Although this paper provides definitive information for doctors and patients, it is necessary to take methodological issues into consideration. First, patients with hydrosalpinx fall into one of the groups which may undergo the freeze-all approach in the methodology of this paper. However, only polycystic ovary syndrome and ovulatory disorders were mentioned in the exclusion criteria. Did the investigators perform FET in the first menstrual cycle after oocyte collection? If not, please clarify in which menstrual cycle these patients underwent FET after oocyte collection; whether pretreatment (salpingectomy or ligation) is performed before FET [3]; and whether this proportion of patients should be included in the comparison of the characteristics of the study population. Second, the endometrial preparation part of the article mentioned that when the endometrial thickness reaches 8 mm or more, endometrial transformation can be performed. However, in the hierarchical analysis of the endometrium, the comparison among groups included two groups of endometrial thicknesses less than 8 mm (including groups less than 6 mm and 7–7.9 mm). Did these two groups of patients undergo pretreatment to increase endometrial thickness prior to FET (such as pretreatment of transvaginal estrogen medication)? Third, the total number of denominators for “pregnancy rate” in Table 4 should be consistent with the number of grouping cases. However, the total number of denominators for Cycle 1 is 525, which is smaller than the actual number of grouping cases which is 526. Similarly, the total number of denominators for Cycle ≥ 2 in the table (450 cases) is also one less than the number in the actual group.