Impact of embryo quality and endometrial thickness on implantation in natural cycle IVF

PURPOSE: The aim of this study is to assess the effect of the endometrial thickness and embryo quality on the implantation potential in natural cycle IVF (NC-IVF). METHODS: A retrospective single-center study was performed on 552 single embryo transfers after NC-IVF. The ‘quality' of the embryos was evaluated trough the number and regularity of blastomeres, degree of fragmentation, and nuclear content of cells. Endometrial thickness was measured in millimeters with transvaginal ultrasound on the day of hCG application. RESULTS: Our findings showed a statistically significant difference in successful implantation until a plateau of 10 mm is reached (p = 0.001). Only one pregnancy was achieved where endometrial thickness was less than 7 mm, and this resulted in an early miscarriage. The predictors of favorable implantation were fragmentation (≤ 10%, p < 0.05) and the number of blastomeres (preferably 8-cell, p < 0.01) on day 3. Embryo quality (R = 0.052) and endometrial thickness (R = 0.18) were closely related to pregnancy rate. The overall implantation rate per embryo transfer was 18.8%. CONCLUSIONS: Embryo quality and endometrial thickness have a significant impact on implantation in NC-IVF. Highest implantation potential has an 8-cell embryo with ≤ 10% fragmentation in the third day following oocyte retrieval. Endometrial thickness of at least 7 mm seems to be the optimal edge of successful pregnancy.