Postpartum hemorrhage (PPH) is defined as excessive bleeding following childbirth, typically quantified as a cumulative blood loss of 1,000 milliliters or more within 24 hours after a cesarean section, or 500 milliliters or more after a vaginal delivery, especially when accompanied by signs of low blood volume. PPH is the leading cause of maternal mortality worldwide, accounting for over 20% of all maternal deaths globally. Medical professionals utilize the “Four T’s”—Tone, Tissue, Trauma, and Thrombin—to categorize the underlying causes, though most cases are not predictable based on history alone. Identifying conditions and events that increase the likelihood of this complication is crucial for preparation and prevention. Risk factors are broadly categorized by their relation to the delivery process, placental integrity, or the mother’s overall health status.
Factors Related to the Labor and Delivery Process
The most common reason for PPH is uterine atony, which is the failure of the uterus to contract sufficiently after the placenta has separated. After birth, the uterine muscle fibers normally contract to compress the blood vessels that supplied the placenta, acting as living ligatures to stop the bleeding. When the muscle loses its tone, these vessels remain open, leading to persistent hemorrhage.
Uterine fatigue, which makes the muscle susceptible to atony, can result from labor that is either prolonged or precipitous (too rapid). Medications used to induce or augment labor, such as oxytocin, can also desensitize the uterine muscle, hindering its ability to contract effectively after delivery.
Mechanical strain from overdistension also compromises muscle function. Conditions like multiple gestation (twins or triplets), excessive amniotic fluid (polyhydramnios), or a very large baby (macrosomia) stretch the uterine muscle fibers beyond their optimal contractile capacity. Additionally, magnesium sulfate, often administered to treat high blood pressure or prevent seizures, acts as a smooth muscle relaxant that can directly inhibit uterine contraction.
The delivery process itself can introduce the risk of trauma-related PPH. Surgical deliveries, such as a cesarean section, involve an incision into the uterine wall and are generally associated with a higher volume of blood loss. Instrumental deliveries, which use forceps or a vacuum device, increase the likelihood of lacerations to the cervix, vagina, or perineum, causing significant bleeding even if the uterus is contracting normally.
Risks Stemming from Placental Implantation and Structure
Issues with the placenta’s attachment or separation fall under the “Tissue” component of PPH causes. If a portion of the placenta or its membranes remains attached to the uterine wall, this retained tissue physically obstructs the muscle fibers from fully clamping down on the bleeding vessels. This retained tissue prevents the uterus from completing its necessary post-delivery contraction (involution), causing immediate or delayed hemorrhage.
A more severe structural risk is the placenta accreta spectrum (PAS), where the placenta implants abnormally deep into the uterine wall. This abnormal attachment means the placenta cannot naturally separate, and attempts to remove it can trigger severe hemorrhage, often requiring an emergency hysterectomy. The spectrum is classified by the depth of invasion: placenta accreta (adherence to the muscle layer), placenta increta (invasion into the muscle), and placenta percreta (invasion through the muscle and potentially into adjacent organs).
The incidence of PAS is strongly linked to a history of previous cesarean sections, as the placenta may implant over the prior surgical scar tissue. Placenta previa, where the placenta covers the opening of the cervix, also significantly amplifies the risk for severe PPH if complicated by any degree of PAS. Deliveries complicated by PAS can result in blood loss exceeding 3,000 milliliters.
Pre-existing Maternal Health Conditions
A mother’s general health history contributes to PPH risk, primarily relating to the “Thrombin” component—the body’s ability to clot blood. A history of PPH in a previous pregnancy is one of the strongest predictors of recurrence, making it a crucial factor in antenatal risk assessment. This prior event suggests an underlying susceptibility that may manifest again.
Pre-existing or acquired clotting disorders, known as coagulopathies, directly interfere with the body’s natural defense mechanism against blood loss. Inherited conditions like von Willebrand disease compromise the coagulation cascade. Furthermore, pregnancy complications such as preeclampsia or HELLP syndrome can lead to acquired coagulopathies by consuming platelets and clotting factors, leaving the mother vulnerable to uncontrolled bleeding.
Severe anemia before delivery also increases the danger associated with PPH, as a patient with a low baseline blood count has a smaller reserve to tolerate even moderate blood loss. Advanced maternal age, defined as 35 years or older at delivery, is another independent risk factor. This may be due to decreased uterine muscle tone combined with a higher incidence of other obstetric complications in this age group.
Obesity is linked to an elevated risk of PPH due to several compounding factors. Increased body mass index (BMI) is associated with prolonged labor, which contributes to uterine atony. Obesity also increases the likelihood of needing a cesarean section, which is itself a risk factor for greater blood loss and placental implantation abnormalities.