What is the management strategy for differentiating between threatened preterm labor and preterm labor?

Differentiating Between Threatened Preterm Labor and Preterm Labor

The differentiation between threatened preterm labor and preterm labor requires assessment of cervical changes, contraction patterns, and risk factors, with cervical length measurement by transvaginal ultrasound being the most reliable diagnostic tool. 1, 2

Diagnostic Criteria

Threatened Preterm Labor

• Defined as regular uterine contractions before 37 weeks' gestation without cervical changes 2
• Patients may present with:
  • Regular contractions (at least 3 per 30 minutes) 3
  • No significant cervical dilation or effacement 2
  • Cervical length >2 cm on transvaginal ultrasound 4
  • Negative fetal fibronectin test (if performed) 4

True Preterm Labor

• Defined as regular uterine contractions with progressive cervical changes before 37 weeks' gestation 5
• Diagnostic criteria include:
  • Regular contractions (at least 3 per 30 minutes) 3
  • Cervical dilation ≥2 cm and/or significant effacement 4
  • Cervical length ≤2 cm on transvaginal ultrasound 4
  • Positive fetal fibronectin test (if performed) 4
  • Ruptured membranes (in some cases) 6

Diagnostic Algorithm

1. Initial Assessment

   • Document frequency and duration of contractions 2
   • Assess for risk factors for preterm birth (previous preterm birth, multiple gestation, cervical insufficiency) 5
   • Check for signs of infection, placental abruption, and fetal well-being 1

2. Cervical Examination

   • Perform digital cervical examination to assess dilation and effacement 2
   • Note: Digital examination may be difficult in women with skeletal dysplasia or anatomical variations 6

3. Transvaginal Ultrasound

   • Measure cervical length - this is the most reliable diagnostic tool 2
   • Cervical length ≤10 mm strongly suggests true preterm labor 3
   • Cervical length 11-30 mm requires additional testing 3

4. Biomarker Testing

   • Fetal fibronectin testing for women with cervical length 11-30 mm 3
   • Positive result (≥50 ng/mL) suggests higher risk of preterm delivery 6

5. Management Decision

   • If cervical changes present (dilation ≥2 cm or length ≤2 cm) → Diagnose as true preterm labor 4
   • If no cervical changes but contractions persist → Diagnose as threatened preterm labor 2

Management Based on Diagnosis

Threatened Preterm Labor

• Observe for 2-4 hours to assess for progression 1
• If no cervical changes occur and contractions subside:
  • Discontinue any initiated tocolysis 6
  • Patient may be discharged with precautions 1
  • Consider repeat cervical length assessment if patient reaches 35-37 weeks 6

True Preterm Labor

• Initiate tocolysis if <34 weeks (to allow for corticosteroid administration) 5, 7
• Administer corticosteroids between 24-34 weeks 8
• Consider magnesium sulfate for neuroprotection if <32 weeks 8, 9
• Continue GBS prophylaxis if indicated 6
• Transfer to facility with appropriate neonatal care if needed 2

Common Pitfalls to Avoid

• Relying solely on contraction frequency without assessing cervical changes 2
• Failing to use transvaginal ultrasound for cervical length measurement 2
• Prolonged tocolysis beyond 48 hours without evidence of benefit 5
• Administering multiple courses of corticosteroids, which may be harmful 5
• Using amoxicillin-clavulanic acid for antibiotic prophylaxis, which increases risk of necrotizing enterocolitis 8
• Delaying diagnosis and treatment of intraamniotic infection due to absence of maternal fever 1

Special Considerations

• For women with threatened preterm labor who have unknown GBS status, obtain vaginal-rectal swab for culture 6
• In women with threatened preterm labor who have a positive GBS screen within the preceding 5 weeks, initiate GBS prophylaxis 6
• Discontinue antibiotics given for GBS prophylaxis if the patient is determined not to be in true labor 6
• For women with skeletal dysplasia, cervical examination may be difficult, and standard management of preterm labor may need modification 6

By following this systematic approach, clinicians can more accurately differentiate between threatened preterm labor and true preterm labor, allowing for appropriate management decisions that optimize maternal and neonatal outcomes.