Postpartum Hemorrhage: Pathophysiology, Causes, and Risk Factors
Postpartum hemorrhage (PPH) is fundamentally a failure of normal hemostatic mechanisms after delivery, with uterine atony accounting for approximately 70% of cases, followed by genital tract trauma (17%), retained placenta (16%), abnormal placentation (4%), and coagulopathy (3%). 1, 2
Definition and Epidemiology
PPH is defined as any hemorrhage associated with signs or symptoms of hypovolemia within 24 hours of delivery, regardless of delivery type 1. It affects 1-5% of all deliveries and represents one of the top three causes of maternal death in the United States, accounting for 27% of maternal deaths worldwide 1. The incidence has increased from 2.7% in 2000 to 4.3% in 2019, representing a 2.6% average annual increase 3.
PPH is classified temporally as:
- Primary (early) PPH: Within first 24 hours after delivery
- Secondary (late/delayed) PPH: From 24 hours to 6 weeks postpartum 1
Pathophysiology
The pathophysiology of PPH centers on disruption of normal postpartum hemostasis through four primary mechanisms:
Normal Hemostatic Mechanisms
After placental separation, the uterus normally contracts to compress spiral arteries and reduce blood flow to the placental bed. This mechanical compression, combined with local thrombosis at vessel sites, prevents excessive bleeding. Failure of any component of this process results in hemorrhage.
The Four T's Framework
1. Tone (Uterine Atony) - 70.6% of cases 2
- Most common cause, representing failure of myometrial contraction after delivery
- The uterus remains boggy and fails to compress uterine blood vessels
- Diagnosed clinically in >75% of patients 1
2. Trauma (Genital Tract Injury) - 16.9% of cases 2
- Lower genital tract lacerations
- Perineal or vaginal hematomas
- Uterine rupture or scar dehiscence
- Subfascial/prevesical hematomas from epigastric vessel injury or rectus muscle tears
- Bladder flap hematomas 1
3. Tissue (Retained Products) - 16.4% of cases 2
- Retained placental tissue preventing adequate uterine contraction
- Adherent placenta (placenta accreta spectrum)
- May be associated with superimposed infection 1
4. Thrombin (Coagulopathy) - 2.7% of cases 2
- Pre-existing coagulation disorders
- Acquired coagulopathies (DIC, dilutional coagulopathy)
- Consumptive coagulopathy from massive hemorrhage
Critical caveat: 7.8% of women have multiple concurrent causes of PPH 2, which supports the use of treatment bundles rather than single interventions.
Risk Factors
Risk factors are stratified by strength of association based on the most recent comprehensive meta-analysis 2:
Strong Association (OR >2.0)
Antepartum factors:
- Previous postpartum hemorrhage (strongest predictor)
- Anemia
- Placenta previa
- Multiple pregnancy
- No antenatal care
- Assisted reproductive technology use
Intrapartum/delivery factors:
- Cesarean birth
- Macrosomia (birthweight >4500g)
- Shoulder dystocia
- Sepsis
- Female genital mutilation 2
Moderate Association (OR 1.5-2.0)
- BMI ≥30 kg/m²
- COVID-19 infection
- Gestational diabetes
- Polyhydramnios
- Pre-eclampsia
- Antepartum hemorrhage 2
Weak Association (OR 1.0-1.5)
- Black and Asian ethnicity
- BMI 25-29.9 kg/m²
- Asthma
- Thrombocytopenia
- Uterine fibroids
- Antidepressant use
- Induction of labor
- Instrumental birth
- Premature rupture of membranes 2
Temporal Trends
The proportion of deliveries with at least one PPH risk factor increased from 18.6% to 26.9% between 2000-2019 3. However, a critical pitfall is that PPH can occur in patients without identifiable risk factors 4, necessitating universal prophylaxis strategies.
Special Considerations
Vascular Uterine Anomalies (VUA)
Myometrial VUA represent subinvolution of the placental bed and may be associated with retained products of conception. Peak systolic velocity >83 cm/s on Doppler ultrasound indicates increased risk of significant PPH. Most resolve with removal of retained tissue or expectant management 1.
Hospital and Regional Variations
Large bed size hospitals, urban teaching facilities, and hospitals in the West and Midwest United States show increased PPH risk, while private investor-owned hospitals demonstrate decreased risk 5. These variations likely reflect differences in patient populations and obstetric practice patterns rather than quality of care alone.
Secondary PPH Causes
Late PPH (24 hours to 6 weeks) is most commonly caused by uterine atony, vascular uterine anomalies, placental attachment disorders, and rarely gestational trophoblastic disease 1.