Background: The use of laparoscopic salpingostomy to treat tubal ectopic pregnancy (TEP) is increasing due to patient preference for subsequent spontaneous pregnancy. However, the incidence of persistent trophoblasts (PT) ranges from 5–29%, although PT rarely occurs after laparoscopic salpingectomy. For this reason, the selection of laparoscopic salpingostomy should be performed carefully. Here, we aimed to identify the risk factors for PT, focusing on serum hCG levels. Methods: We reviewed the medical records of 128 patients who underwent laparoscopic surgeries for TEP, including 62 with laparoscopic salpingostomy and 66 with laparoscopic salpingectomy. Among these cases, we identified 13 with PT detected during the postoperative follow-up period. We performed a multivariate logistic regression analysis to assess the influence of each representative factor, particularly serum hCG levels, on the occurrence of PT. We first performed this analysis for 62 cases with laparoscopic salpingostomy and then for all 128 cases. Additionally, we focused on the reduction rates of serum hCG levels after surgery. Based on the scatter plot of the association between the reduction rates and elapsed time after surgery, we tried to create linear regression lines for estimating the appropriate postoperative follow-up period. Results: In the multivariate analyses of the 62 cases with laparoscopic salpingostomy, “high hCG”, including serum hCG levels of 2000 and 4000 mIU/mL, showed significance. These results were also detected in the analysis of all 128 cases. Next, we obtained the regression lines based on the scatter plots of the association between reduction rates of serum hCG levels and elapsed time after surgery. By referring to the slopes of the regression lines, we could predict 37 and 30 days as the necessary time for serum hCG levels to decrease to one-thousandth in the cases with laparoscopic salpingostomy and salpingectomy. Conclusions: Our evaluation of the association between serum hCG level and PT allows us to provide the selection criteria, namely, serum hCG levels over 2000 and 4000 mIU/mL, for laparoscopic salpingostomy. Additionally, our analysis of the association between the reduction rates of serum hCG levels and the elapsed time after surgery allows us to predict the appropriate length for the postoperative follow-up period.