Postpartum hemorrhage is a significant loss of blood after giving birth, and it’s the number one reason for maternal morbidity and maternal death around the world.
Specifically it’s defined as losing more than 500ml of blood after a vaginal delivery or more than 1000ml after a cesarean section delivery.
Of course, deliveries can be messy and it’s impossible to measure the precise amount of blood that’s lost, and there’s the possibility of internal bleeding.
So additional criteria to consider for postpartum hemorrhage include a decrease of 10% or more in hematocrit from baseline, as well as changes in the mother’s heart rate, blood pressure, and oxygen saturations —all of which suggest a significant blood loss.
Significant bleeding in the first 24 hours after delivery is called primary postpartum hemorrhage, and after that it’s called secondary, or late, postpartum hemorrhage.
The most common causes of postpartum hemorrhage can be lumped into four groups which can easily be remembered as the “4 Ts”: Tone, Trauma, Tissue, and Thrombin.
Tone refers to a lack of uterine tone, also known as uterine atony—basically a soft, spongy, boggy uterus, and this is the main cause of postpartum hemorrhage, generally resulting in a slow and steady loss of blood.
Now, the uterus is a muscular organ wrapped by three layers of smooth muscle called the myometrium, which contracts during labor to dilate and efface the cervix and ultimately push out the fetus and placenta.
After delivery, the myometrium continues to contract and this squeezes down on the placental arteries at the point where they are attached to the uterine wall, which clamps them shut, and therefore reduces uterine bleeding.