Received November 11, 2023
Accepted May 12, 2024

Objective: This study examined whether blastocysts transferred on day 5 or day 6 of embryo development, as well as positivity for anti-thyroid peroxidase antibodies, affect gestational outcomes in euthyroid women undergoing in vitro fertilisation. Methods: Of 428 women who underwent in vitro fertilisation assessed in this retrospective cohort study, 212 (49.5%) underwent embryo transfer on day 5 of blastulation and 216 (50.5%) on day 6. Dichotomization based on anti-thyroid peroxidase antibodies status was also performed, with 370 (86.4%) women testing negative and 58 (13.6%) testing positive. Clinical and hormonal data and rates of clinical pregnancy, miscarriage, and live births were compared between the groups. Results: When evaluating gestational outcomes based on the day of blastulation, a statistically significant difference was observed in clinical pregnancy rates [51.4% (day 5) vs. 40.7% (day 6); p=0.033]. However, there was no significant difference in the relative frequencies of miscarriages (p=1.000), live births (p=1.000), or preterm births (p=1.000). Using Cramer's V test, a weak association was found between the day of blastulation and clinical pregnancy outcomes (V2=10.7%; p=0.027). There were no statistically significant differences between the anti-thyroid peroxidase antibodies-negative and -positive groups in terms of clinical pregnancy rates (P =0.396), miscarriages (P =0.129), and live births (P =0.129). Conclusions: Higher rates of clinical pregnancy were observed in women who underwent embryo transfers performed on day 5 compared to those on day 6. However, no effect was observed with gestational outcomes. Further, anti-thyroid peroxidase antibody positivity did not have a statistically significant impact on gestational outcomes.