Does delayed implantation affect reproductive outcomes in assisted reproductive technology?

Background To investigate whether delayed implantation affect reproductive outcomes in assisted reproductive technology (ART). Methods This is a single-centre retrospective cohort study. All frozen-thawed cylces were performed between February 2022 to January 2024. Serum hCG levels were sequentially tested on blastocyte transfer day 6, 8, 10 and 12. Patients with initial serum hCG ≥ 5IU/L on transfer day 6 were recorded as the normal implantation and hCG ≥ 5IU/L detected on transfer day 8, 10 and 12 was recorded as the delayed implantation. The reproductive outcomes were compared between groups. The primary outcome was ongoing pregnancy rate, secondary outcomes were biochemical pregnancy rate, miscarriage rate and ectopic pregnancy rate. Results For patients with delayed implantation, the biochemical pregnancy rate was significantly higher in contrast to those with normal implantation (52.6% vs. 16.1%, P = 0.000).The early miscarriage rate was comparable between groups (5.3% vs. 9.9%, P = 0.345). The ectopic pregnancy rate was also higher in delayed implantation group ( 3.5% vs. 0.3%, P = 0.006). The ongoing pregnancy rate was significantly lower in delayed implantation group in contrast to normal implantation group (31.6% vs. 73.6%, P = 0.000). After multivariate logistic regression analyses, normal implantation group had increased odds of ongoing pregnancy compared to delayed implantation group (adjusted OR, 6.36; 95% CI, 3.46–11.70, P = 0.000). Conclusions The delayed implantation is strongly associated with higher incidence of biochemical pregnancy and ectopic pregnancy as well as decreased ongoing pregnancy rate.