Endometrial thickness is a good predictor of clinical pregnancy and live birth rates in fresh and frozen-thawed embryo transfer cycles

OBJECTIVE: The aim of this study was to investigate the effect of the endometrial thickness (EMT) measured on embryo transfer day on clinical pregnancy (CPR), live birth (LBR), and miscarriage rates (MR) in fresh and frozen-thawed embryo transfer cycles. PATIENTS AND METHODS: This prospective cohort study consisted of 160 patients, 80 frozen-thawed and 80 fresh cycles. Endometrial thickness was measured on the day of embryo transfer for fresh and frozen cycles. In addition to the endometrial thickness, the endometrial appearances of the patients in both groups were also recorded. Those without trilaminar appearance were excluded from the study. Both groups were classified according to the EMT values measured on the day of the transfer. The number of groups was calculated considering 1 mm intervals of EMT, and a total of 8 groups were formed. The initial group started with <6 mm, while the final group was >12 mm. The relationship between endometrial thickness, clinical pregnancy, live birth and miscarriage rates was analyzed using multivariable regression analysis. RESULTS: A significant correlation was observed between endometrial thickness values, clinical pregnancy rates, live birth rates in the analyses performed after adjusting for age, infertility duration, body mass index, number of MII oocytes, number and quality of embryos transferred. Based on univariate analysis, each 1 mm increase in EMT resulted in a significant increase in CPR (OR=1.08, 95% CI: 1.07-1.09, p<0.01). Similarly, the increase in EMT led to a significant increase in LBR (OR=1.12, 95% CI: 1.10-1.14, p< 0.01). Although the relationship between miscarriage rates and EMT is not as clear as LBR and CPR, the increase in EMT led to a significant reduction in MR (OR=1.05, 95% CI: 1.03-1.05, p= 0.03). The lowest CPR was detected at EMT <6 mm, while the EMT value with the highest CPR was 11-12 mm in both groups. Likewise, in both groups, the lowest LBR was detected at EMT <6 mm, while the EMT value with the highest LBR was 11-12 mm. Although MR showed a fluctuating course according to EMT values, it reached its highest rate at EMT < 6 mm (100%). In EMT 11-12 mm, MR reached its lowest level (12.5%). If EMT >12 mm, an increase in MR rates was observed again (33.3%). CONCLUSIONS: Clinical pregnancy and live birth rates remain optimal if the endometrial thickness is between 11-12 mm in both fresh and frozen-thawed cycles. A fluctuating course is observed between EMT values and miscarriage rates.