Postpartum hemorrhage is caused by the uterus failing to contract after delivery in about 70% of cases. The remaining cases stem from tears in the birth canal, retained placental tissue, or blood clotting problems. It’s defined as blood loss exceeding 500 mL after a vaginal delivery or 1,000 mL after a cesarean section, and it can happen within the first 24 hours or develop weeks later.
How the Uterus Normally Stops Bleeding
Understanding what goes wrong starts with understanding what’s supposed to happen. During pregnancy, the placenta is fed by a dense network of blood vessels in the uterine wall. When the placenta detaches after birth, those vessels are essentially torn open. The body’s primary defense is for the uterine muscle to clamp down hard, physically compressing those blood vessels shut. This contraction is the single most important mechanism that stops postpartum bleeding.
When that contraction fails, or when something else interferes with the process, hemorrhage follows. Clinicians organize the causes into four categories, sometimes called the “four Ts”: tone, trauma, tissue, and thrombin.
Uterine Atony: The Most Common Cause
Uterine atony, meaning the uterus stays soft and boggy instead of contracting, accounts for roughly 70% of all postpartum hemorrhage. Without that muscular squeeze, the spiral arteries that supplied the placenta continue to bleed freely.
Several conditions make atony more likely, and they share a common theme: anything that stretches the uterus beyond its normal capacity or exhausts the muscle before delivery. Carrying twins or triplets, excess amniotic fluid, or a baby weighing over 4,000 grams (about 8 pounds 13 ounces) all overdistend the uterine wall. A very long labor can fatigue the muscle, while a very fast labor may not give it time to adapt. Infection of the amniotic membranes during labor also weakens uterine contractions. Even certain medications, including magnesium sulfate (used for preeclampsia) and some general anesthetics, can relax the uterus enough to contribute.
Women who have delivered five or more babies have a higher risk because the uterine muscle has been stretched repeatedly over the years. Prolonged use of synthetic oxytocin during labor induction can also desensitize the uterus, making it less responsive to the natural oxytocin surge after delivery.
Trauma to the Birth Canal
About 20% of postpartum hemorrhage cases result from physical injury during delivery. This includes tears (lacerations) of the cervix, vagina, or perineum, as well as blood collecting in the tissue (hematomas) that may not be immediately visible. In rare cases, the uterus itself can rupture, particularly in women with a prior cesarean scar.
Severe vaginal or perineal lacerations are a well-documented independent risk factor for hemorrhage. Assisted deliveries using forceps or vacuum increase the likelihood of these injuries. A large baby passing through a relatively small birth canal raises the risk as well. The bleeding from trauma can be just as brisk as bleeding from atony, but it requires a different response since the uterus may be contracting normally while the bleeding comes from a torn vessel elsewhere.
Retained Placental Tissue
Even small fragments of placenta or membranes left inside the uterus can prevent it from contracting fully, leading to hemorrhage. Retained tissue accounts for about 10% of cases. Sometimes the issue is obvious at delivery, with an incomplete placenta visible on inspection. Other times, small fragments go undetected and cause bleeding hours or weeks later.
A more serious version of this problem is placenta accreta spectrum, where the placenta grows too deeply into the uterine wall and cannot separate cleanly after birth. This condition is becoming more common, partly due to rising cesarean delivery rates, since prior uterine surgery is one of its main risk factors. When the placenta is attached to the front wall of the uterus, the risk of severe hemorrhage and the need for blood transfusions is higher compared to when it attaches to the back wall.
Blood Clotting Disorders
Clotting problems cause roughly 1% of postpartum hemorrhage, but they can be among the most dangerous because bleeding may be widespread and difficult to control. A hallmark sign is bleeding not just from the uterus but from IV sites, gums, or any break in the skin.
Some women have inherited conditions that impair clotting, such as von Willebrand disease. Others develop clotting problems during pregnancy or labor. Severe preeclampsia, HELLP syndrome (a complication involving liver and blood cell damage), placental abruption, fetal death in utero, and amniotic fluid embolism can all trigger a chain reaction where the body uses up its clotting factors faster than it can replace them. This creates a dangerous cycle where the very act of bleeding makes it harder for the body to stop bleeding.
Secondary Hemorrhage: Bleeding After the First Day
Not all postpartum hemorrhage happens right after delivery. Secondary hemorrhage occurs between 24 hours and 12 weeks postpartum. It’s rarer than primary hemorrhage but can catch families off guard because they’ve already left the hospital.
The three main causes are retained tissue, infection of the uterine lining (endometritis), and a condition called subinvolution of the placental site. Subinvolution means the blood vessels at the spot where the placenta was attached fail to shrink back to their pre-pregnancy size. They remain abnormally dilated in the uterine wall, and weeks after delivery, they can bleed heavily and without warning. In some cases, a weakened artery at the placental site can balloon outward, forming a pseudoaneurysm that eventually ruptures.
Endometritis typically comes with fever and uterine tenderness alongside the bleeding, which helps distinguish it from other causes. Retained tissue may also present with fever if the fragments become infected.
Risk Factors That Increase Your Chances
Some risk factors overlap with specific causes, but a few deserve attention on their own because they can be identified before or during labor:
- History of postpartum hemorrhage. A previous episode is one of the strongest predictors of it happening again.
- Fetal macrosomia. Babies over 4,000 grams stretch the uterus and increase the chance of both atony and birth canal trauma.
- Pregnancy-induced hypertension. Preeclampsia and related conditions raise the risk through multiple pathways, including clotting problems and liver dysfunction.
- Assisted reproductive technology. Pregnancies conceived through IVF or similar methods carry a higher hemorrhage risk, partly because they’re more likely to involve multiples or placental abnormalities.
- Excessive weight gain. Gaining more than 15 kilograms (about 33 pounds) during pregnancy has been linked to higher hemorrhage rates.
- Older maternal age and grand multiparity. Both are independently associated with increased risk.
It’s worth noting that postpartum hemorrhage sometimes occurs in women with no identifiable risk factors at all. That unpredictability is one reason hospitals routinely use preventive measures for every delivery.
How Hemorrhage Is Prevented
The standard preventive approach is called active management of the third stage of labor, meaning the period between the baby’s birth and delivery of the placenta. The most important component is a dose of oxytocin given by injection shortly after the baby’s shoulders are delivered. This synthetic version of the body’s natural contraction hormone encourages the uterus to clamp down before significant bleeding can start.
This practice is recommended for all women regardless of risk level. Studies consistently show it reduces the rate of postpartum hemorrhage compared to waiting passively for the placenta to deliver on its own. For cesarean deliveries, oxytocin is given intravenously instead. Beyond medication, gentle traction on the umbilical cord and uterine massage after placental delivery are additional steps that help the uterus contract and stay firm.