Massalha M, Faranish R, Romano S, et al. - The relationship between IVC diameter and postpartum blood loss was examined and the utility of the variations in IVC diameter, if present, as a marker in diagnosis of postpartum blood loss investigated. In this prospective case control study, women with singleton pregnancy that delivered vaginally at term comprised the study cohort. Women with a visually estimated blood loss of 1000ml or more at time of enrollment form the study group. Women with uneventful fourth stage of labor form the control group. Measurement of IVC diameters was done by the use of abdominal ultrasonography during inspiration (IVCi) and expiration (IVCe) and calculation of the collapsibility index (CI) was done (IVC e-i/IVC e). The final analysis was performed including 36 and 72 women in the study and control groups, respectively. Relative to the control group, the study group had significantly smaller IVCi and IVCe diameters. Per findings, change in IVC diameters occur in response to postpartum blood loss. They suggest measurement of IVC diameters as an objective and valuable method for the diagnosis of blood loss and prediction of cases that might require blood transfusion.