Severe postpartum hemorrhage (PPH) is defined as an estimated blood loss of more than 1000 mL after delivery because this volume reflects significant hemorrhage that can lead to hypovolemia, shock, and increased maternal morbidity. The threshold of 1000 mL helps clinicians identify patients requiring urgent resuscitation and potential surgical intervention, such as uterine artery ligation or hysterectomy. Blood loss below this volume (e.g., 500 mL) is considered primary PPH but usually less critical. Prompt recognition of >1000 mL loss ensures early management and reduces adverse outcomes.

Reference: Williams Obstetrics, Cunningham FG, 25th Edition.