What are the potential causes and management options for vaginal bleeding in a pregnant patient?

Differential Diagnosis of Vaginal Bleeding in Pregnancy

Immediate Diagnostic Framework by Trimester

The differential diagnosis and management of vaginal bleeding in pregnancy is fundamentally determined by gestational age, with ultrasound imaging being the cornerstone of evaluation before any digital examination is performed. 1, 2

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First Trimester Bleeding (0-13 weeks)

Most Common Diagnoses

• Threatened abortion (viable intrauterine pregnancy with bleeding) is the most frequent outcome, occurring in 7-27% of pregnancies, with approximately 12% overall miscarriage risk 3, 4
• Ectopic pregnancy represents 7-20% of pregnancy of unknown location cases and must be excluded immediately as it can rapidly progress to life-threatening hemorrhage 2, 3, 5
• Spontaneous abortion/early pregnancy loss encompasses incomplete, complete, missed, or inevitable abortion depending on cervical os status 4, 6
• Subchorionic hematoma with viable pregnancy 2, 3
• Gestational trophoblastic disease is less common but critical to exclude 3

Less Common First Trimester Causes

• Cervical lesions, polyps, or cervicitis are typically identified after ultrasound excludes serious pathology 2, 3
• Uterine arteriovenous malformation can present with first trimester bleeding, requiring Doppler imaging for diagnosis, though many resolve spontaneously 3

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Second and Third Trimester Bleeding (14+ weeks)

Life-Threatening Diagnoses to Exclude First

• Placenta previa is the most common diagnosis made in late pregnancy bleeding, affecting approximately 1 in 200 pregnancies at delivery 1, 2
• Placental abruption affects approximately 1% of pregnancies and is associated with worse perinatal outcomes, though ultrasound identifies at most 50% of cases 1, 2
• Vasa previa carries risk of fetal exsanguination and requires Doppler evaluation, as vessels overlying the internal cervical os can be subtle 2
• Uterine rupture is particularly important in patients with prior cesarean delivery, looking for myometrial disruption or lower uterine segment thickness <2.5 mm 2

Physiologic and Benign Causes

• "Bloody show" with term labor is normal physiologic bleeding characterized as pink or brown-tinged mucus, minimal in volume 1, 7
• Cervical change with preterm labor can cause bleeding 1
• Cervical polyps, cervicitis, or vaginal lacerations are typically identified by history and physical examination after imaging excludes placental causes 1, 6

Approximately Half Have No Identifiable Cause

• Bleeding for no identifiable reason is seen in approximately 50% of those with vaginal bleeding in pregnancy 1

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Critical Diagnostic Algorithm

Step 1: Obtain Ultrasound BEFORE Digital Examination

Digital pelvic examination is absolutely contraindicated until ultrasound has definitively excluded placenta previa, low-lying placenta, and vasa previa, as examination before imaging can precipitate catastrophic hemorrhage. 1, 2, 3

• Transvaginal ultrasound is the primary diagnostic tool for first trimester evaluation, providing superior resolution compared to transabdominal ultrasound 2, 3
• Transabdominal and transvaginal ultrasound with Doppler are usually appropriate for second and third trimester evaluation 1, 2
• Transvaginal ultrasound is safe even with placenta previa and provides superior visualization of the cervix and lower uterine segment 2

Step 2: Obtain Quantitative Beta-hCG Level

• Obtain quantitative beta-hCG level regardless of ultrasound findings in first trimester bleeding to help identify ectopic pregnancies and guide interpretation 2, 3
• Serial beta-hCG measurements every 48 hours are required until diagnosis is established if no intrauterine pregnancy is visualized 2, 3
• Repeat transvaginal ultrasound when beta-hCG reaches discriminatory threshold of 1,500-2,000 mIU/mL, at which point a normal intrauterine pregnancy must show a gestational sac 2, 3

Step 3: Interpret Ultrasound Findings

For First Trimester:

• If intrauterine pregnancy with fetal cardiac activity is confirmed, ectopic pregnancy is essentially ruled out (except rare heterotopic pregnancy in <1% of spontaneous conceptions) 2, 3
• Most pregnancies of unknown location (80-93%) will be early intrauterine or failed intrauterine pregnancies, but 7-20% will be ectopic, requiring vigilant follow-up 2, 3
• Schedule follow-up ultrasound in 1-2 weeks for continuing pregnancies with subchorionic hematoma 2, 3

For Second and Third Trimester:

• Assess placental location and measure exact distance from internal cervical os or degree of overlap 2
• Evaluate for placental abruption, though ultrasound sensitivity is limited 1, 2
• Assess cervical length, as short cervix with bleeding significantly increases preterm delivery risk 2
• Perform Doppler evaluation for vasa previa 2

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Critical Pitfalls to Avoid

• Ultrasound misses up to 74% of ectopic pregnancies initially and up to 50% of placental abruptions, making serial beta-hCG monitoring and clinical suspicion critical when initial ultrasound is non-diagnostic 2, 3
• Normal vital signs do not exclude significant placental pathology, as placental abruption can present with normal hemodynamics initially but rapidly deteriorate 2, 7
• Avoid overinterpretation of single ultrasound that could lead to inappropriate treatment with methotrexate or dilation and curettage, potentially harming a normal early pregnancy 3
• Never perform digital cervical examination before ultrasound if bleeding increases beyond minimal spotting, as this can precipitate catastrophic hemorrhage if placenta previa or vasa previa is present 2, 7

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Long-Term Implications

• First trimester bleeding is associated with increased risk of preterm delivery, placental abruption later in pregnancy, and small for gestational age infants 2, 3, 7
• Placental abruption is associated with recurrence risk in subsequent pregnancies, increased risk of stillbirth, and potential for fetal growth restriction 2