Study selection
By searching MEDLINE, SCIE and EMBASE, 219 articles were found after eliminating duplicate citations. After reading the full text, ninety-one citations were retained. Next, the following citations were excluded: thirty-nine were not intended to describe and predict the outcome of PUL, twelve had insufficient data integrity to support the analysis, nine did not have high-risk category (i.e., EP or PPUL), and two did not propose specific prediction protocols. The flow of paper retrieval is summarized in figure 1. As a result, twenty nine studies were included.7, 10-37 Among them, three used M1 model, four used single progesterone threshold protocol, two used M6 model, nine used M4 model, one used P1 model, five used hCG ratio (hCG 48 hours / hCG at 0 hours) protocol, two used the two steps method, the combination of progesterone and hCG threshold protocol, seven used single hCG cut-off protocol, one used FFN protocol, one used ultrasound imaging results and hCG, one used ultrasound imaging results and one used Activin-A protocol.