Placenta Previa vs Placental Abruption: How to Tell Them Apart
Both placenta previa and placental abruption present with antepartum vaginal bleeding in the second or third trimester and share several risk factors. The core axis that separates them is pain and uterine tone: previa causes painless bleeding with a soft uterus, while abruption causes painful bleeding with a rigid, tender uterus and fetal distress.
How to tell them apart
| Feature | Placenta previa | Placental abruption |
|---|---|---|
| Underlying pathology | Placenta implanted over or near the internal cervical os | Premature separation of a normally implanted placenta |
| Pain | Painless bleeding | Painful bleeding, with uterine tenderness |
| Uterine tone | Soft, nontender uterus | Rigid, tender uterus with high-frequency contractions |
| Character of bleeding | Bright red, episodic; first episode often self-limited, subsequent episodes can be more severe | May be concealed (hidden behind the placenta), so external bleeding can underestimate blood loss |
| Fetal status | Fetus typically stable unless hemorrhage is severe | Fetal distress is part of the classic triad; severe cases can cause fetal death |
| Signature risk factors | Prior uterine surgery, multiparity, multiple gestation, in addition to shared factors | Hypertension (chronic and preeclampsia) is the #1 risk factor; also cocaine use, trauma, prior abruption, thrombophilias |
| Maternal coagulation | Not associated with DIC | Can progress to DIC in severe cases |
| Diagnostic approach | Confirmed with ultrasound (transabdominal then transvaginal) | A clinical diagnosis; ultrasound is insensitive and misses most abruptions |
The reasoning
Anchor on pain and uterine tone. Painless bleeding with a soft uterus points to previa; painful bleeding with a rigid, tender uterus plus fetal distress is the classic triad of abruption. Use risk factors to arbitrate borderline cases: hypertension, cocaine, or trauma strongly favor abruption, whereas the setting of a known low-lying placenta favors previa. Remember that abruption is a clinical diagnosis—do not let a normal ultrasound talk you out of it. When previa is suspected, avoid digital cervical examination, which can provoke catastrophic hemorrhage.
Key tests
- Ultrasound: reliably localizes the placenta over or near the os in previa, but is poor for abruption (only about 25% are visible), so a negative scan does not exclude abruption.
- Fetal heart monitoring: typically reassuring in previa unless bleeding is severe, but shows distress (e.g., minimal variability, late decelerations) in abruption.
- Coagulation studies/labs: help identify DIC, which can complicate severe abruption but is not a feature of previa.
What they share
- Vaginal bleeding in the second or third trimester
- Prior cesarean section is a risk factor for both
- Advanced maternal age is a risk factor for both
- Smoking is a risk factor for both
- Preterm cases managed with hospitalization, steroids for fetal lung maturity, and monitoring with readiness to deliver
Pitfalls
- Relying on ultrasound to exclude abruption—most abruptions are not visible, so diagnosis is clinical.
- Performing a digital cervical exam when previa is suspected, which can trigger massive hemorrhage.
- Underestimating blood loss in abruption because bleeding may be concealed behind the placenta with little external bleeding.
- Forgetting that prior cesarean section, smoking, and advanced maternal age are shared risk factors and cannot by themselves distinguish the two.
- Overlooking placenta accreta spectrum risk: prior C-section plus placenta previa is the highest-risk combination for abnormal placental invasion.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.