Postpartum hemorrhage (PPH) is excessive bleeding after childbirth, defined as losing more than 500 milliliters of blood after a vaginal delivery or more than 1,000 milliliters after a cesarean. It is the leading direct cause of maternal death worldwide, responsible for roughly one-quarter of maternal deaths annually. The risk factors fall into several categories, some identifiable well before labor begins and others that emerge during delivery itself.
Why Most Cases Happen: Uterine Atony
The single most common cause of PPH is uterine atony, which means the uterus fails to contract firmly after the placenta is delivered. Normally, those contractions clamp down on open blood vessels at the placental site. When the uterus stays soft and relaxed instead, bleeding continues unchecked. Several factors make atony more likely:
- An overdistended uterus. Carrying twins, triplets, or more stretches the uterus beyond its typical capacity. The same applies to delivering a large baby (a condition called macrosomia) or having an excess of amniotic fluid. A uterus that has been stretched significantly has a harder time contracting effectively.
- Prolonged or very rapid labor. A labor that drags on for many hours can exhaust the uterine muscle. Paradoxically, an extremely fast (precipitous) delivery can also leave the uterus unable to contract properly afterward.
- Labor induction or augmentation. Using medications to start or speed up contractions can tire out the uterine muscle by the time delivery is complete.
- Magnesium sulfate treatment. This medication, commonly given for severe preeclampsia, relaxes smooth muscle throughout the body, including the uterus. It works by lowering calcium levels inside muscle cells, which directly reduces the uterus’s ability to contract.
- Infection of the amniotic sac (chorioamnionitis). Inflammation from infection weakens uterine muscle function. In one large study of cesarean deliveries performed after labor began, chorioamnionitis was present in about 20% of women who experienced severe hemorrhage compared to 12% of those who did not.
- Uterine fibroids. These noncancerous growths can interfere with the uterus’s ability to contract uniformly.
- Five or more previous deliveries. Each pregnancy stretches the uterine muscle further, and after many pregnancies the muscle may not bounce back as effectively.
Placental Problems
The placenta is supposed to separate cleanly from the uterine wall after the baby is born. When it doesn’t, the exposed blood vessels at the attachment site keep bleeding. Retained placental tissue, where fragments remain inside the uterus, is one of the more common causes of both immediate and delayed hemorrhage.
More dangerous are conditions where the placenta grows too deeply into the uterine wall. Placenta accreta, where placental tissue invades the muscle of the uterus, makes clean separation impossible and often causes severe bleeding. A history of prior cesarean delivery increases this risk because scar tissue in the uterine wall can allow abnormal placental attachment in future pregnancies.
Placenta previa, where the placenta covers the cervix, and placental abruption, where the placenta separates from the wall too early, both carry significant hemorrhage risk. Abruption is closely linked to high blood pressure disorders during pregnancy.
Trauma During Delivery
Physical injury to the birth canal is the second major category of PPH causes. Cesarean delivery itself carries the highest surgical bleeding risk. Women undergoing a cesarean after labor has already begun face a higher rate of hemorrhage (about 3.1%) compared to those who have a planned cesarean before labor starts (about 2%). The combination of laboring and then requiring surgery compounds the bleeding risk.
Instrument-assisted vaginal delivery, using forceps or a vacuum device, increases the chance of lacerations to the cervix, vagina, or perineum. Episiotomy, a surgical cut to widen the vaginal opening, adds another potential bleeding source. A baby in a persistent face-up position during delivery also raises the likelihood of tissue injury.
Blood Clotting Disorders
Some women enter pregnancy with an inherited condition that impairs their blood’s ability to clot. Von Willebrand disease is the most common inherited bleeding disorder, and women who have it face a recognized risk of serious postpartum bleeding. Low levels of clotting factors mean that even normal amounts of bleeding from the placental site or small tears can become difficult to control.
Clotting problems can also develop during pregnancy itself. Severe preeclampsia and its more dangerous variant, HELLP syndrome (which involves the breakdown of red blood cells, elevated liver enzymes, and low platelet counts), directly impair the body’s clotting system. Placental abruption can trigger a chain reaction that consumes clotting factors faster than the body can replace them. In rare cases, amniotic fluid entering the bloodstream can cause a sudden, severe clotting failure.
Maternal Characteristics That Raise Risk
Certain characteristics present before pregnancy or labor begins can place a woman in a higher risk category. Obesity increases the chance of PPH through multiple pathways: it’s associated with larger babies, longer labors, and higher rates of cesarean delivery. Advanced maternal age is an independent risk factor as well, though the exact threshold varies by guideline.
Anemia before delivery matters more than many people realize. Women who enter labor with low hemoglobin levels have less margin for blood loss. One study found that a pre-delivery hemoglobin level at or below 9.9 grams per deciliter tripled the odds of severe hemorrhage during cesarean delivery compared to women with levels above 11.
Having a previous postpartum hemorrhage is itself a risk factor. Women who bled heavily after one delivery are more likely to experience it again in subsequent pregnancies, which is why clinicians flag this in prenatal records.
Bleeding That Starts Days or Weeks Later
Not all postpartum hemorrhage happens in the delivery room. Secondary PPH refers to significant bleeding that occurs between 24 hours and six weeks after delivery. The causes differ from immediate hemorrhage. In one study, uterine infection (endometritis) accounted for about two-thirds of secondary hemorrhage cases, while retained placental tissue was responsible for roughly one in five. Less common causes include breakdown of a cesarean scar, abnormal blood vessel formations at the surgical site, and undiagnosed clotting disorders.
Cesarean delivery creates its own set of delayed risks. Pseudoaneurysms, where a weakened artery near the surgical site balloons and eventually bleeds, occurred exclusively in women who had cesarean deliveries. Vaginal delivery, on the other hand, carried unique risks for delayed bleeding from birth canal injuries that weren’t fully recognized at the time of delivery.
How Risk Is Assessed Before Delivery
Hospitals increasingly use a three-tiered classification system, sorting women admitted for delivery into low, medium, or high risk categories for PPH. The American College of Obstetricians and Gynecologists recommends this approach so that the right preparations, including having blood products available and extra staff on standby, are in place before bleeding begins rather than after.
A woman with no prior cesarean, a single baby, no bleeding disorder, and no placental abnormalities would typically fall into the low-risk category. Medium risk might include someone with a prior cesarean, a large baby, or a history of PPH. High-risk classification is reserved for conditions like placenta accreta, active bleeding disorders, or multiple simultaneous risk factors. For all women regardless of risk tier, standard practice now includes giving a medication to help the uterus contract immediately after delivery, along with controlled techniques to deliver the placenta, both of which significantly reduce the chance of hemorrhage.