Painless Vaginal Bleeding at 8 Months Pregnancy
The most common cause of painless vaginal bleeding at 8 months (third trimester) is placenta previa, affecting approximately 1 in 200 pregnancies at delivery, and immediate transabdominal ultrasound is the preferred initial diagnostic procedure to exclude this and other life-threatening conditions before any digital pelvic examination is performed. 1
Critical Initial Management Principle
Digital pelvic examination must be avoided until placenta previa, low-lying placenta, and vasa previa have been definitively excluded by ultrasound imaging. 1 This is non-negotiable as examination could precipitate catastrophic hemorrhage if placenta previa is present.
Primary Differential Diagnoses for Painless Bleeding
Most Common Causes (in order of frequency):
- Placenta previa - The most frequently diagnosed pathologic cause, occurring in approximately 1 in 200 deliveries 1
- Vasa previa - Much less common but potentially catastrophic, affecting 1 in 2,500 to 1 in 5,000 deliveries 1
- "Bloody show" - Physiologic bleeding from cervical change as labor approaches, which is expected and normal 1
- No identifiable cause - Seen in approximately 50% of cases with vaginal bleeding in pregnancy 1
Less Common Causes:
- Cervical causes: cervicitis, cervical polyp, cervical erosion 1, 2
- Vaginal lacerations 1
- Low-lying placenta (placenta near but not covering the internal os) 1
Diagnostic Approach Algorithm
Step 1: Immediate Ultrasound Evaluation
Transabdominal ultrasound is the mandatory first imaging study and should specifically evaluate: 1
- Placental location and inferior placental margin
- Distance from placental edge to internal cervical os (or degree of overlap if covering the os)
- Placental umbilical cord insertion site
- Cervix visualization from external to internal os
- Any vessels overlying the internal cervical os (critical for vasa previa diagnosis)
Step 2: Transvaginal Ultrasound (if needed)
Transvaginal ultrasound should be performed when transabdominal imaging is inconclusive or inadequate - this has been proven both accurate and safe even with placenta previa. 1 It provides superior visualization of:
- The exact relationship between placenta and internal cervical os
- Measurement of placental overlap (≥15 mm overlap predicts persistent previa; ≥25 mm at 20-23 weeks indicates exceptional cesarean risk) 1
- Subtle vessels that may indicate vasa previa
Step 3: Doppler Ultrasound Enhancement
Color and spectral Doppler velocimetry is invaluable as an adjunct for: 1
- Identifying vessels overlying the internal cervical os (vasa previa)
- Distinguishing fetal arterial vessels from maternal vessels
- Detecting subtle vascular abnormalities that may be missed on grayscale imaging alone
High-Risk Features Requiring Hospitalization
The following warrant admission for extended observation beyond initial evaluation: 1
- Persistent or recurrent bleeding
- Confirmed placenta previa
- Confirmed vasa previa
- Any abnormal fetal heart rate patterns
- Uterine contractions
Management Based on Diagnosis
If Placenta Previa Confirmed:
- Pelvic rest is mandatory (no intercourse, no vaginal examinations) 3
- Hospitalization if significant bleeding occurs 3
- Planned cesarean delivery 1
If Vasa Previa Identified:
- This represents a true obstetric emergency with risk of fetal exsanguination
- Immediate obstetric consultation and likely hospitalization 3
- Planned cesarean delivery before membrane rupture
If "Bloody Show" (Physiologic):
- This is normal cervical change preceding labor 1
- No intervention required if otherwise reassuring
Critical Pitfalls to Avoid
Never perform digital examination before ultrasound exclusion of placenta previa - this could trigger massive hemorrhage 1, 4
Do not rely on ultrasound to exclude all pathology - approximately half of bleeding cases have no identifiable cause even after complete evaluation 1
Vasa previa can be subtle - specifically look for and use Doppler to identify any vessels crossing the internal os, as this condition is often missed 1
Transabdominal ultrasound alone may be inadequate - suboptimal bladder filling or maternal symphysis shadowing can obscure the lower uterine segment, necessitating transvaginal approach 1