Purpose <p>Monitoring early human chorionic gonadotropin (hCG) levels is crucial for predicting pregnancy outcomes in anxious couples undergoing assisted reproductive technology. The aim of this study was to compare subsequent reproductive outcomes between normal and delayed implantation of embryos based on early sequential human hCG tests.</p> Methods <p>Patients with initial serum hCG ≥ 5&#xa0;IU/L on transfer day 6 were recorded as the normal implantation and hCG ≥ 5&#xa0;IU/L detected on transfer days 8, 10 and 12 was recorded as delayed implantation. A total of 455 eligible patients enrolled in the normal implantation group and 60 patients were enrolled in the delayed implantation group. The reproductive outcomes were analyzed between groups.</p> Results <p>The delayed implantation group exhibited significantly lower hCG levels compared with normal implantation group on day 8 (17.7 ± 16.0 vs. 77.4 ± 43.9, <i>P</i> = 0.000), day 10 (40.7 ± 31.5 vs. 182.8 ± 117.1, <i>P</i> = 0.000) and day 12 (87.0 ± 113.5 vs. 495.1 ± 346.3, <i>P</i> = 0.000). The hCG rising ratio within 48&#xa0;h was comparable when day 10 compared with day 8 between two groups (2.4 ± 1.6 vs. 2.3 ± 0.8, <i>P</i> = 0.322) and significantly decreased when day 12 compared with day 10 in the delayed implantation group (1.9 ± 2.4 vs. 2.5 ± 1.2, <i>P</i> = 0.002). The biochemical pregnancy rate was significantly higher in the delayed implantation group (61.7% vs. 15.4%, <i>P</i> = 0.000).The early miscarriage rate was comparable between groups (5.0% vs.10.1%, <i>P</i> = 0.249). The ectopic pregnancy rate was also significantly higher in the delayed implantation group (3.3% vs. 0.4%, <i>P</i> = 0.016). The ongoing pregnancy rate was significantly reduced in the delayed implantation group in contrast to normal implantation group (30.0% vs. 74.1%, <i>P</i> = 0.000).</p> Conclusions <p>Delayed implantation is strongly associated with higher incidence of biochemical pregnancy and ectopic pregnancy as well as decreased ongoing pregnancy rate.</p>