Abstract: Objective：To discuss the causes，clinical characteristics, influence factors and preventions of undiagnosed postpartum hemorrhage（UDPPH）． Methods: 1 528 cases of vaginal delivery from January 2013 to December 2017 were retrospectively analyzed，and were divided into three groups: UDPPH group, postpartum hemorrhage (PPH) group and control group(those cases with neither UDPPH nor PPH). The clinical characteristics and influence factors of UDPPH group were analyzed and compared with those of PPH group． Results: The incidence of UDPPH and PPH was 10.5% and 6.1% of vaginal deliveries, respectively. The antepartum hemoglobin of UDPPH group and PPH group were both lower than that of the control group. The gravidity, the total duration of labor, duration of the third stage of labor, the incidence of retained placenta and perineal trauma were all higher than those of the control group (P＜0.05), with statistical differences between them. The rates of primiparity and episiotomy in UDPPH group were higher than those of the control group (P＜0.05), and there were statistical differences between them. But there were no significant differences in the rates of primiparity and episiotomy between PPH group and control group. The rates of primiparity and episiotomy were the influence factors those UDPPH were different from PPH. Conclusions: UDPPH is frequent among women giving birth vaginally. To reduce the incidence，we need to strengthen the observation after delivery，estimate blood loss accurately and prevent it according to the influence factors.

Key words: Postpartum hemorrhage, Episiotomy, Labor stage, third