Differences in Implantation Timing After Embryo Transfer Based on Embryo Characteristics

Abstract

Objective: To evaluate the differences in implantation timing after Embryo Transfer (ET) based on embryo characteristics. Patients: This retrospective study evaluated cases of 7,269 frozen-thawed ET cycles using single blastocysts that resulted in single live birth. The implantation timings after ET were speculated by a calculation formula made for the dynamics of serum human chorionic gonadotropin (HCG) changes. The patients were divided into two groups based on the initial serum HCG (10 days after ET): high-HCG group (HCG > 800) and low-HCG group (HCG < 50). Results: The average estimated implantation timing (HCG = 0) was 3.3 days after ET. The serum HCG levels after logarithmic conversion (logHCG) changed in parallel regardless of the starting HCG levels. The estimated implantation timing varied approximately five days (2.5–7.5 days after ET) based on the starting HCG levels. The ratios of matured blastocysts (3 or higher in Gardner’s grading scale) were significantly higher in the high-HCG group (90.9%) than in the low-HCG group (47.4%). Similarly, the average grade of blastocysts was higher in the high-HCG group than in the low-HCG group (4.72 vs. 3.15, p < 0.01). Conclusions: The implantation timing after ET varies based on embryo characteristics.