The Association Between Early Progesterone Rise and Serum Estradiol Levels as Well as Endometrial Thickness in IVF Cycles.

Background/Objectives: The success of artificial reproductive technologies (ARTs) depends on different factors, such as patient-specific reproductive features, ovarian response to stimulation, oocyte and embryo quality, and endometrial receptivity. This study aimed to evaluate their association with oocyte yield, fertilization, endometrial thickness, and pregnancy outcomes. Methods: A prospective clinical study included 128 women undergoing IVF/ICSI. Baseline hormone levels (E2, P4, FSH, LH, AMH) were assessed prior to stimulation. E2 levels were monitored during stimulation, and P4 was measured on the day of oocyte retrieval. Patients were grouped based on P4 levels (<2 ng/mL vs. ≥2 ng/mL). IVF outcomes and endometrial characteristics were statistically analyzed. Results: Lower P4 levels (<2 ng/mL) on the day of oocyte retrieval were significantly associated with higher fertilization rates (p < 0.003), more fertilized oocytes (p < 0.001), and increased pregnancy rates (p < 0.001). Elevated P4 (≥2 ng/mL) correlated with a higher frequency of thin endometrium (<7 mm, p < 0.007). E2 levels on the hCG trigger day correlated positively with the number of retrieved and mature oocytes and fertilization outcomes (p < 0.05). Patients who achieved pregnancy had lower P4 and BMI, and higher E2, AMH, and endometrial thickness. ROC identified a P4 threshold of 1.99 ng/mL with moderate predictive value. Conclusions: Elevated progesterone levels on the day of oocyte retrieval negatively impact fertilization and pregnancy outcomes, likely due to impaired endometrial receptivity. Combined assessment of P4, E2, AMH, and endometrial thickness may enhance embryo transfer planning and improve IVF success rates.