Purpose: This study sought to evaluate whether or not the sonographic measurement of the inferior vena cava (IVC) diameter predicts severe bleeding in women with postpartum hemorrhage (PPH). It further compared the efficacy of the IVC diameter, shock index, and serum lactate levels in this prediction. Methods: This prospective study included female patients with ≥500 mL of bleeding after vaginal delivery between September 1, 2019 and May 31, 2021. The IVC diameter during inspiration (IVCmin) and expiration (IVCmax), shock index and serum lactate levels of the patients were measured. The patients were divided into two groups according to the postpartum vaginal blood loss [non-severe PPH and severe PPH (SPPH)]. After multivariate logistic regression analysis revealed the significant parameters, we constructed the receiver operating characteristic (ROC) curves to determine their power in predicting SPPH. Results: Of the 201 patients included in the study, SPPH developed in 21.4% of the population, while non-severe PPH occurred in 78.6%. The area under the curve (AUC) values ​​of shock index, lactate, IVCmin and IVCmax for SPPH prediction were 0.772, 0.791, 0.851 and 0.874, respectively. According to the ROC analysis, at the cut-off value of 7.3 mm, IVCmax had 90.7% sensitivity and 70.8% specificity in predicting SPPH. IVCmax was independently associated with SPPH (Odds ratio: 0.74, 95% CI: 0.63–0.86). Conclusion: We found that the ultrasonographic measurement of IVC diameter is more valuable than the shock index and lactate parameters in the early detection of SPPH. Therefore, IVC diameter may be a useful prognostic marker for SPPH.