4.3 Third-Trimester Bleeding & Amniotic Fluid Volume Disorders
Key Takeaways
- Painless bright red bleeding with a soft uterus suggests placenta previa, painful dark bleeding with a rigid tender uterus suggests abruption, and painless bleeding at membrane rupture with fetal bradycardia or a sinusoidal pattern suggests vasa previa.
- Digital cervical examination is absolutely contraindicated in third-trimester bleeding until ultrasound has excluded placenta previa.
- Ultrasound is negative in about half of placental abruptions, so abruption remains a clinical diagnosis and imaging must never delay delivery.
- Polyhydramnios is an amniotic fluid index of 24 cm or more or a deepest vertical pocket of 8 cm or more; oligohydramnios is an index below 5 cm or a pocket below 2 cm.
- Prenatally diagnosed vasa previa is delivered by planned cesarean at 34 0/7 to 37 0/7 weeks, and amniotomy is never performed.
Last updated: September 2026
Third-Trimester Vaginal Bleeding Differential
Vaginal bleeding in the late second or third trimester is a life-threatening obstetric emergency until proven otherwise. Immediate assessment must prioritize maternal hemodynamic stability and fetal oxygenation while identifying the etiology.
| Clinical Characteristic | Placenta Previa | Placental Abruption (Abruptio Placentae) | Vasa Previa |
|---|---|---|---|
| Underlying Anatomy | Placenta overlies or is adjacent to internal cervical os | Premature detachment of a normally implanted placenta | Fetal vessels course through membranes over the internal os |
| Nature of Bleeding | Painless, bright red vaginal bleeding | Painful, dark red vaginal bleeding (20% concealed) | Painless vaginal bleeding occurring at membrane rupture |
| Uterine Tone & Contractions | Uterus is soft, nontender, and relaxed; no tachysystole | Uterus is firm, board-like, exquisitely tender; high-frequency contractions | Uterine tone is normal; no tenderness or hypertonus |
| Fetal Heart Rate (FHR) | Usually reassuring initially unless massive maternal shock | Variable decelerations, late decelerations, bradycardia, loss of variability | Catastrophic fetal bradycardia or sinusoidal FHR pattern |
| Primary Risk Factors | Prior cesarean birth, prior previa, multiparity, advanced maternal age, smoking | Chronic hypertension, preeclampsia, cocaine use, trauma, sudden uterine decompression | Velamentous cord insertion, bilobed/succenturiate placenta, in vitro fertilization |
| Diagnostic Standard | Transvaginal ultrasound (safe and highly accurate) | Clinical diagnosis; ultrasound is negative in 50% (does not rule out abruption) | Color Doppler transvaginal ultrasound showing pulsating vessels |
| Mandatory Contraindication | Strictly NEVER perform a digital vaginal exam until previa is excluded | Do not delay emergency delivery for ultrasound confirmation | Never perform artificial rupture of membranes (AROM) |
Placenta Previa
- Classification: Complete (placental tissue completely covers the internal os) or Marginal (placental edge lies within 2 cm of the internal os). If the edge is >2 cm from the os in the lower uterine segment, it is termed a low-lying placenta.
- Digital Examination Precaution: Digital cervical examination can tear placental cotyledons from the lower uterine segment, inducing catastrophic maternal exsanguination. Ultrasound confirmation of placental location is mandatory prior to any digital exam in bleeding patients.
- Delivery Timing: Stable patients with persistent complete placenta previa undergo planned cesarean delivery between 36 0/7 and 37 6/7 weeks gestation to avoid the onset of unscheduled labor and severe hemorrhage.
Placental Abruption
- Pathophysiology: Rupture of maternal spiral arterioles in the decidua basalis produces a retroplacental hematoma that shears the placenta from the uterine wall. Impaired gas exchange leads to acute fetal hypoxemia and acidemia.
- Clinical Spectrum: May be revealed (external vaginal bleeding) or concealed (blood remains trapped behind the placenta; uterine fundal height increases). The release of tissue thromboplastin into the maternal circulation frequently triggers rapid-onset disseminated intravascular coagulation (DIC) characterized by hypofibrinogenemia (<200 mg/dL), thrombocytopenia, and elevated D-dimer.
- Management: Large-bore IV access (two 16-gauge catheters), blood bank crossmatch (4 units packed RBCs, fresh frozen plasma, platelets, and cryoprecipitate), continuous FHR monitoring, and emergency delivery (cesarean birth unless rapid vaginal delivery is imminent and fetal status is stable).
Vasa Previa
- Mechanism: Fetal vessels, unprotected by Wharton's jelly or placental parenchyma, traverse the membranes across the cervical os ahead of the fetal presenting part. When membranes rupture (spontaneously or via AROM), the vessels lacerate, resulting in rapid fetal exsanguination (the total blood volume of a term fetus is only ~250–300 mL).
- Management: If diagnosed prenatally via color Doppler, elective cesarean is performed at 34 0/7 to 37 0/7 weeks. If acute rupture occurs, immediate emergent delivery within minutes is the sole chance of neonatal survival.
Amniotic Fluid Volume Disorders
Amniotic fluid volume reflects fetal renal perfusion, swallowing, lung fluid secretion, and placental membrane exchange. It is quantified via ultrasound using either the Amniotic Fluid Index (AFI) (sum of deepest vertical pockets in four quadrants) or the Single Deepest Vertical Pocket (DVP).
Polyhydramnios
- Diagnostic Thresholds: AFI ≥24 cm or DVP ≥8 cm.
- Etiologies: Idiopathic (50%–60%), maternal diabetes (fetal osmotic diuresis), fetal gastrointestinal anomalies impairing swallowing (tracheoesophageal fistula, esophageal or duodenal atresia), fetal neuromuscular disorders (impaired swallowing), congenital infections (parvovirus, CMV), fetal hydrops, and twin-to-twin transfusion syndrome (recipient twin).
- Clinical Complications: Maternal dyspnea, premature rupture of membranes, preterm labor, and abnormal fetal lie. Intrapartum risks include umbilical cord prolapse upon membrane rupture and placental abruption caused by rapid decompression of the overdistended uterus.
Oligohydramnios
- Diagnostic Thresholds: AFI <5 cm or DVP <2 cm.
- Etiologies: Rupture of membranes (most common), uteroplacental insufficiency (chronic hypertension, preeclampsia, systemic vascular disease, severe FGR), fetal genitourinary malformations (bilateral renal agenesis [Potter sequence], posterior urethral valves), post-term pregnancy (>41–42 weeks), and medications (ACE inhibitors, indomethacin).
- Clinical Complications: Pulmonary hypoplasia (if present in early second trimester due to absence of fluid splinting alveoli), umbilical cord compression (manifesting as variable decelerations on FHR tracings), meconium aspiration syndrome, and intrauterine fetal death.
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Test Your Knowledge
A 33-year-old G3P2 at 32 4/7 weeks gestation arrives at the obstetric triage unit via private vehicle reporting a sudden onset of painless, bright red vaginal bleeding that soaked two sanitary pads over the past hour. Her vital signs are: BP 114/72 mmHg, HR 82 bpm, RR 18 breaths/min, SpO2 99% on room air. Abdominal palpation reveals a soft, nontender, relaxed uterus. The continuous electronic fetal heart rate monitor demonstrates a baseline of 140 bpm with moderate variability and no decelerations. What is the most critical initial action?
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