Third-Trimester Bleeding: Previa & Abruption
A board-focused breakdown of the two classic causes of third-trimester bleeding — painless placenta previa vs. painful placental abruption — covering risk factors, presentation, the no-vaginal-exam-before-ultrasound rule, and next-best-step management, plus vasa previa and placenta accreta spectrum as don't-miss differentials.
The two must-know third-trimester bleeds
Bleeding after 20 weeks has two board-favorite culprits: placenta previa and placental abruption. The reflex discriminator: painLESS bleeding = previa; painFUL bleeding with a tender, contracting uterus = abruption.
The single most important rule before touching the patient: NO digital or speculum vaginal exam until placenta previa is excluded by ultrasound — probing a previa can trigger catastrophic hemorrhage.
First moves for any unstable third-trimester bleed are identical: two large-bore IVs, CBC, type and crossmatch, coagulation studies (including fibrinogen), continuous fetal monitoring, and check maternal Rh status — give Rho(D) immune globulin if Rh-negative.
- Placenta previa = placenta implants over/near the internal cervical os
- Low-lying placenta = edge within 2 cm of the os but not covering it (updated terms; old "complete/partial/marginal" retired)
- Previa risk factors: prior cesarean, prior previa, multiparity, advanced maternal age, multiple gestation, smoking
- Abruption = premature separation of a normally implanted placenta
- Abruption's leading associations: hypertension (chronic HTN, preeclampsia) and cocaine; also trauma, smoking, PPROM, and prior abruption (strongest predictor of recurrence)
- Up to ~20% of abruptions are concealed — minimal external bleeding despite severe separation, so visible blood underestimates true loss
- Previa blood is classically bright red; abruption blood is often dark
Previa vs. abruption at a glance
| Feature | Placenta previa | Placental abruption |
|---|---|---|
| Pain | Painless | Painful |
| Uterus | Soft, non-tender | Tender, firm/hypertonic |
| Bleeding | Bright red, external | Dark; may be concealed |
| Fetal distress | Uncommon early | Common |
| Key risk factor | Prior cesarean | Hypertension / cocaine |
| Diagnosis | Transvaginal ultrasound | Clinical (US insensitive) |
| Vaginal exam | Contraindicated until US | Avoid until previa excluded |
| DIC risk | Low | High |
| Delivery | Cesarean | Depends on stability |
Vignette: A 30-year-old G3P2 at 32 weeks with two prior cesareans has sudden painless, bright red vaginal bleeding. Uterus is soft and non-tender; fetal tracing is reassuring; vitals stable.
- Most likely dx: placenta previa
- Next best step: transabdominal → transvaginal ultrasound (TVUS is safe and confirmatory; do NOT perform a digital exam)
- Management: preterm and stable → expectant management with pelvic rest and observation; betamethasone if <34 weeks for fetal lung maturity; anti-D if Rh-negative. Plan scheduled cesarean at 36–37 weeks (earlier for recurrent or heavy bleeding).
- Don't miss: prior cesarean + previa → screen for placenta accreta spectrum.
Vignette: A 34-year-old G4P3 at 36 weeks with chronic hypertension develops painful vaginal bleeding after cocaine use. Uterus is tender and rigid with frequent contractions; blood is dark; tracing shows late decelerations.
- Most likely dx: placental abruption
- Next best step: treat as a clinical diagnosis — do NOT rely on ultrasound (low sensitivity; a normal US does not exclude it). Continuous fetal monitoring, large-bore IV access, type & cross, and check coags/fibrinogen for DIC.
- Management: non-reassuring tracing, maternal instability, or a term fetus → prompt delivery. Emergency cesarean for fetal/maternal compromise; vaginal delivery is acceptable if delivery is imminent or in fetal demise (mother stable). Transfuse blood/products as needed.
- Painless = Previa; Painful = abruPtion — the single highest-yield discriminator (the P in abruPtion cues painful).
- Abruption = the 3 A's: Abdominal pain, Angry (rigid, tender, hypertonic) uterus, Abnormal BP (hypertension/preeclampsia). Remember external bleeding may be concealed, so it underestimates true blood loss.
- Previa = "don't Probe": no digital or speculum exam until ultrasound locates the placenta — probing can provoke catastrophic hemorrhage.
- Abruption complications: DIC (release of placental tissue thromboplastin/tissue factor), hemorrhagic shock, fetal hypoxia/demise, Couvelaire uterus (blood extravasating into the myometrium), acute kidney injury
- Previa + prior cesarean(s): rising risk of placenta accreta spectrum — deliver at a center prepared for massive hemorrhage and possible hysterectomy
- Vasa previa (the curveball): painless bleeding at rupture of membranes + fetal bradycardia or a sinusoidal fetal heart tracing; the blood is fetal, so exsanguination is rapid. Diagnose antenatally by ultrasound; planned cesarean before labor (~34–37 weeks)
- Bloody show and early labor are benign mimics — but only entertained after previa is excluded
Board next-step algorithm
For any third-trimester bleed: (1) Assess stability — vitals, two large-bore IVs, labs, type & cross. (2) Start continuous fetal monitoring. (3) Get an ultrasound before any vaginal exam to locate the placenta. (4) If US shows previa → no exam; choose expectant management vs. cesarean by gestational age and stability. (5) If US is unrevealing and the uterus is tender/rigid → treat as abruption clinically. (6) Give Rho(D) immune globulin if Rh-negative; after trauma, use a Kleihauer–Betke test to quantify fetomaternal hemorrhage and guide anti-D dosing.
At any point, non-reassuring fetal status or maternal instability → deliver.
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