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Abstract Study question Is there any difference in ongoing pregnancy rate in women with thin endometrium (8 mm) when scrutinized based on estradiol levels on triggering day? Summary answer Either with a low, moderate, or high estradiol level on the day of triggering, ongoing pregnancy rates do not change in patients with thin endometrium. What is known already Endometrial thickness has been argued to be one of the proxy determinants of endometrial receptivity. In that context, although the optimal thickness has been widely stated to be between 8-14 mm, the negative effect of thin endometrium on pregnancy outcome is still controversial in in-vitro-fertilization (IVF) cycles. Nevertheless, particularly lack of large sample size of women with thin endometrium among the available literature, is the leading factors that avoids to solid conclusion.Furthermore, it might be hypothesized that patients with thin endometrium and accompanying low estradiol might be distinct from women with thin endometrium despite relatively high estradiol level. Study design, size, duration A retrospective cohort study was designed among patients that has undergone in-vitro fertilization (IVF) treatment between January-2018 and June-2023 in Bahceci Health Group. Only patients having an endometrial thickness 8 mm on the day of triggering were recruited. Other inclusion criteria was having at least one blastocyst to be transferred. All frozen-thawed embryo transfer cycles were excluded. Female age and body mass index were not considered as an exclusion criterion. Participants/materials, setting, methods During the study period, a total of 3076 IVF cycles with fresh ET were employed. Of them, 814 had an endometrial thickness of 8 mm on the day of triggering. Among 814 IVF cycles, 25th, 50th and 75th quartiles of estradiol levels on the day of triggering referred to 322.8, 755.7 and 1310.5 pg/ml, respectively. The number of IVF-ET cycles in the 1st, 2nd, 3th and 4th quartiles were 203, 204, 204 and 203. Main results and the role of chance The mean (SD) age of women recruited in the final analyses was 36.5 (5.5) years and endometrial thickness was 7.2 (0.8) mm. The pregnancy rates per ET with regard to 1st, 2nd, 3th and 4th quartiles were 37.6%, 33.0%, 37.3% and 44.3%, respectively without any statistical significance (p = 0.134). However, the respective ongoing pregnancy rates were 26.1%, 18.1%, 22.1%, 31.0 % and the highest quartile had a statistically superior ongoing pregnancy rates than the second (p = 0.003) and third quartiles (p = 0.022). When binary logistic regression analysis was performed by inserting the female age, body-mass index, duration of infertility and quartiles of estradiol levels on the day of triggering, only female age remained as an independent factor for the prediction of an ongoing pregnancy (RR: 0.92 CI95%: 0.87 to 0.98, p = 0.008). Utilization of estradiol level as a continuous variable in the same model also failed to present the independent validity of it to predict an ongoing pregnancy. Notably, the lowest thickness of a positive pregnancy resulting with an ongoing pregnancy was 4.7 mm. Limitations, reasons for caution The inherent risks cannot be excluded due to the retrospective nature of the current study, which had been efforted to overcome with regression analysis. Wider implications of the findings The current study includes one of the largest numbers of experience particularly with thin endometrium on the day of triggering. However, having a thin endometrium despite low or relatively high estradiol level does yield any prediction for the pregnancy outcome. Trial registration number Not Applicable
Bahçeci et al. (Mon,) studied this question.