Background: In assisted reproductive technology (ART) cycles in addition to embryo quality, the receptivity of the endometrium plays the main role in clinical outcomes. Endometrial receptivity is necessary to implantation of an embryo, and ultrasound has been established as an appreciated, simple, and non-invasive technique in the evaluation of the endometrial preparation before embryo transfer in fresh in vitro fertilization (IVF) cycles. Debate on the predictive value measuring endometrial thickness before administering human chorionic gonadotropin (HCG) for ovulation triggering in ART is ongoing. In order to explore the impact of endometrial thickness on triggering day on ongoing pregnancy rate (OPR) in ART cycles, we retrospectively analyzed data from 1000 patients undergoing IVF/ICSI cycles. Results: The data showed pregnancy rate was increased in the endometrial thickness of 8 mm to 11 mm then decreased, and in endometrial thickness (Ent) > 14 mm, pregnancy rate was zero. There were significant differences in endometrial thickness and pregnancy outcomes between different age groups. The pregnancy rate was higher (32%) in 23-30 years old women, and the range of Ent in this group was 6-12 mm. Also, the data showed a positive correlation between Ent with AMH and estradiol levels and the number of COC and MII oocytes and a negative correlation between female age with Ent. Conclusion: The result showed that Ent on hCG administration day is associated with pregnancy outcomes in fresh IVF/ICSI cycles with different age groups, although some of the clinical parameters may have an effect on Ent. Large studies are needed to make a definitive conclusion.