Maternal mortality due to postpartum hemorrhage (PPH) continues to be one of the most important causes of maternal death worldwide. PPH is a significantly underestimated obstetric problem, primarily because a lack of definition and diagnosis. The ‘traditional’ definition of primary PPH based on quantification of blood loss has several limitations. Notoriously, blood loss is not measured or is significantly underestimated by visual estimation and there are no generally accepted cut-offs limits for estimated blood loss. A definition based on hematocrit change is not clinically useful in an emergency such as PPH, as a fall in hematocrit postpartum shows poor correlation with acute blood loss. The need for erythrocyte transfusion alone to define PPH is also of limited value, as the practice of blood transfusion varies widely. Definitions based on symptoms of hemodynamic instability are problematic, as they are late signs of depleted blood volume and commencing failure of compensatory mechanisms threatening the mother's life. There is thus currently no single, satisfactory definition of primary PPH. Proper and timely diagnosis of PPH should above all include accurate estimation of blood loss before vital signs change. Estimation of blood loss by calibrated bags has been shown to be significantly more accurate than visual estimation at vaginal delivery. Careful monitoring of the mother's vital signs, laboratory tests, in particular coagulation testing, and immediate diagnosis of the cause of PPH are important key factors to reduce maternal morbidity and mortality.