Objective: The purpose of this study was to identify risk factors for Adherent placenta.

 Methods: A retrospective cohort study was conducted on 3,733 vaginal delivery of singleton pregnancies between October 2015 and May 2021 at Showa University Northern Yokohama Hospital. The case group included 34 pregnancies which required the manual removal of the placenta and was clinically diagnosed with Adherent placenta. These cases were then compared with a control group consisting of 68 cases randomly selected from pregnancies demonstrating a normal delivery of the placenta. We compared the maternal backgrounds and delivery processes between the two groups. The placental pathology in the case group was also examined.

 Results: According to a multivariate analysis, ART pregnancy and a premature rupture of the membrane were independently associated factors. In the case group, chorioamnionitis was also found in 50% of the cases.

 Conclusion: In addition to ART pregnancies, our findings indicated that a premature rupture of the membrane may be a risk factor for Adherent placenta.