The association between hypercoagulation/fibrinolysis supression and severity of intraventricular hemorrhage (IVH) in preterm infants has not been investigated. The study aimed to determine how the coagulation–fibrinolysis balance prior to IVH onset affects the severity of IVH. Participants included 72 preterm infants born at <33 weeks and 0 days of gestation. Blood-related data were analyzed prior to IVH onset. An ordered logistic regression model was created with gestational week, birth weight, non-reassuring fetal status, chorioamnionitis, fibrinogen, antithrombin activity, platelet count, soluble fibrin, thrombin–antithrombin complex, and plasmin–α2 plasmin inhibitor complex/plasminogen activator inhibitor 1 (PIC/PAI-1) ratio as the explanatory variables and severity of IVH as the objective variable. The Markov chain Monte Carlo method was used to estimate the model’s parameters. The expected value of the partial regression coefficient of the PIC/PAI-1 ratio was estimated to be -0.875, the 95% Bayesian confidence interval ranged from -1.555 to -0.261, and the posterior probability of the coefficient being negative was 99.78%, which can be interpreted as follows: the lower the PIC/PAI-1 ratio in the model, the more severe the IVH. Hypercoagulation/fibrinolysis suppression is a risk factor for severe IVH. Treatment targeting hypercoagulability and low fibrinolytic activity may prevent severe IVH.