Preterm Labor vs. Threatened Preterm Labor

Introduction

Preterm labor and threatened preterm labor are related conditions but have distinct definitions, diagnostic criteria, and management strategies. Let's break them down systematically.

---

1. Definition

---

2. Epidemiology & Risk Factors

Both conditions share common risk factors, but some are stronger predictors of actual preterm birth.

Shared Risk Factors

• Obstetric History: Previous preterm birth, multiple gestation, short cervical length (<25 mm)
• Maternal Factors: Smoking, infection (e.g., urinary tract infection, bacterial vaginosis), anemia, substance use
• Uterine Factors: Polyhydramnios, uterine malformations, placenta previa/abruption
• Fetal Factors: Fetal anomalies, growth restriction, multiple gestation
• Iatrogenic: Assisted reproductive technologies (IVF), cervical procedures (LEEP, cone biopsy)

Higher Risk for True Preterm Labor

• Cervical insufficiency (history of painless dilation in second trimester)
• Short cervix < 25 mm (via transvaginal ultrasound)
• Positive fetal fibronectin (fFN) test

---

3. Clinical Features

---

4. Diagnosis

Preterm Labor: Confirmatory Criteria

• Gestational age 20-37 weeks
• Documented regular uterine contractions
• Cervical changes:
  • Dilation ≥3 cm
  • Effacement ≥80%
  • Progressive cervical change on serial exams

Threatened Preterm Labor: Criteria

• Gestational age 20-37 weeks
• Uterine contractions without cervical change
• Cervical length > 25 mm on transvaginal ultrasound
• Negative fetal fibronectin (fFN) test (high negative predictive value for delivery within 7 days)

Diagnostic Tests for Both Conditions

1. Transvaginal Ultrasound (TVUS)
   • Cervical length ≤25 mm → high risk of preterm labor
   • Cervical funneling → increased risk
2. Fetal Fibronectin (fFN) Test
   • Positive: Suggests disruption of choriodecidual interface, risk of delivery within 7 days
   • Negative: Strongly suggests labor will NOT occur in the next 7-14 days
3. Speculum Exam & Cervical Exam
   • Rule out infections (chorioamnionitis, bacterial vaginosis)
   • Check for premature rupture of membranes (PROM)
4. Tocodynamometry
   • Confirms contraction frequency and regularity

---

5. Management

A. Threatened Preterm Labor

✅ If no cervical change & negative fFN → Expectant Management

• Hydration & rest (to reduce uterine irritability)
• Observation for 4-6 hours (to assess contraction pattern)
• Reassess cervical length with TVUS
• Discharge home if contractions stop, no cervical change, and reassuring fetal monitoring
• Progesterone therapy (if history of preterm birth or short cervix)
• Patient education: Warning signs of preterm labor (e.g., increased contractions, leakage of fluid, vaginal bleeding)

B. Preterm Labor (Confirmed)

✅ If cervical change or positive fFN → Active Management

1. Tocolytics (if <34 weeks)
   • Goal: Delay delivery 48 hours to allow corticosteroids to take effect
   • First-line: Calcium channel blockers (nifedipine)
   • Alternative: Beta-agonists (terbutaline), NSAIDs (indomethacin <32 weeks), Magnesium sulfate (for neuroprotection <32 weeks)
2. Corticosteroids (if <34-36 weeks)
   • Betamethasone (IM 12 mg every 24 hours for 2 doses) or
   • Dexamethasone (6 mg IM every 12 hours for 4 doses)
   • Enhances fetal lung maturity, reduces risk of neonatal respiratory distress syndrome (RDS)
3. Magnesium Sulfate (if <32 weeks)
   • Provides fetal neuroprotection, reduces cerebral palsy risk
4. Antibiotics (if PPROM or GBS unknown)
   • Ampicillin + Azithromycin if preterm premature rupture of membranes (PPROM)
   • Penicillin if GBS status unknown
5. Hospital Admission & Monitoring
   • Continuous fetal monitoring for distress
   • Monitor for chorioamnionitis (maternal fever, tachycardia, uterine tenderness)
6. Delivery Considerations
   • If <34 weeks: Try to delay labor with tocolytics
   • If >34 weeks: Delivery may be considered based on risks/benefits

---

6. Prognosis & Complications

---

7. Key Takeaways

• Preterm labor = Cervical change + Regular contractions
• Threatened preterm labor = Contractions without cervical change
• TVUS & fFN test help differentiate the two
• Threatened preterm labor → Observe, hydrate, follow-up
• Preterm labor → Steroids, tocolytics, magnesium (if <32 weeks), and admission
• Delivery timing is based on gestational age and maternal/fetal risks