Well, it depends on who you ask - ACOG defines it as greater or equal to 1000ml of blood loss within that first 24 hours, but most research articles define it as greater or equal to 500ml, while they define severe postpartum hemorrhage as greater or equal to 1000ml. But what is it actually?
The World Health Organization decided to tackle this question to look at what level of blood loss is the most clinically relevant in a meta-analysis from last year. They reviewed 12 different databases and over 300000 patients to look at levels of blood loss and when that was associated with mortality/severe morbidity. They found that the standard 500ml cutoff was actually only around 75% sensitive, but when they dropped those levels to 300/400/450, they lost a lot of specificity. So, what they did is they came up with their own rules and re-evaluated them based on their data to see how sensitive and specific they were.
What they found with a sensitivity of 87% and a specificity of 66-76% was:
Consider someone as having CLINICALLY SIGNIFICANT blood loss if they had EITHER:
Greater or equal to 300ml of blood loss PLUS abnormal hemodynamics (SBP <100, DBP <60, SI>1, HR>100)
OR
Greater or equal to 500ml of blood loss
So look at the vitals + the blood loss together, and use those to guide your clinical actions!
References
Gallos I, Williams CR, Price MJ, et al. Prognostic accuracy of clinical markers of postpartum bleeding in predicting maternal mortality or severe morbidity: a WHO individual participant data meta-analysis. Lancet. 2025;406(10514):1969-1982. doi:10.1016/S0140-6736(25)01639-3
