To evaluate whether the etiology of thin endometrium impacts clinical pregnancy rate (CPR) in in-vitro fertilization (IVF) cycles. This is a retrospective cohort study of all cycles between 2015-2023 in two university hospitals. All women with an endometrial thickness (ET) of <8 mm before a fresh or frozen embryo transfer were included. Patient characteristics, stimulation cycle characteristics, embryology data, and endometrial preparation methods were recorded. Women with or without history of a dilatation and curettage (D&C) were compared. Patients with other intracavitary procedures, e.g., hysteroscopic myomectomy, delivery and adhesions and patients with any intracavitary lesions were also excluded. Each patient was included with their first IVF cycle. Blastocyst or cleavage stage transfer was done in the absence of elevated serum progesterone levels on the trigger day in fresh cycles or before commencing progesterone in programmed frozen embryo transfer cycles. A total of 392 cycles (191 fresh and 201 frozen-thawed) were included in the analyses. ET was comparable between patients with and without history of D&C (6.78+-0.9 vs. 6.88 +-0.8 mm, p=0.3). While body mass index (BMI) and duration of infertility were similar between the groups, female age was significantly higher (35.8 +-5.6 vs. 32.3 +-5.6, p<.001) in women with a history of D&C as compared with women idiopathic thin endometrium. CPR were similar between the groups. ET was also comparable between patients who had a clinical pregnancy and not. However, after adjusting for female age, women with idiopathic thin endometrium had a lower CPR when compared with women having at least one D&C (OR 0.61, 95%CI (0.37±0.99; p=0.046). The pregnancy outcome of patients with idiopathic thin endometrium on the day of embryo transfer is worse than the women with thin endometrium, possibly secondary to a previous D&C. Although thin endometrium is mostly referred to as an ET below 8 mm on the day of ET, the independent impact on IVF outcome is unclear. However, there is a lack of data on whether patients without any apparent reason for thin endometrium have comparable outcomes with women having thin endometrium but with a history of endometrial intervention. Nevertheless, recent studies do not support ET as an independent predictor for live birth in fresh or frozen-thawed embryo transfer cycles unless an intracavitary pathology was not identified. Our results suggest that a thin endometrium, despite the lack of endometrial intervention, might point to an inherent defect in receptivity compared to women having curettage. However, further studies must confirm whether this finding reflects a genuine primary receptivity failure in that cohort of women. [ABSTRACT FROM AUTHOR]