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

Feature	Preterm Labor	Threatened Preterm Labor
Definition	Regular uterine contractions leading to cervical changes (effacement/dilation) before 37 weeks of gestation	Presence of contractions before 37 weeks but without cervical change

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

Feature	Preterm Labor	Threatened Preterm Labor
Gestational Age	< 37 weeks	< 37 weeks
Contractions	Regular, painful contractions (≥4 in 20 min or ≥8 in 60 min)	Irregular or infrequent contractions
Cervical Change	Yes (dilation ≥ 3 cm or effacement ≥ 80%)	No significant cervical change
Fetal Fibronectin (fFN) Test	Often positive	Can be positive or negative
Cervical Length (TVUS)	≤ 25 mm	Often > 25 mm

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

Transvaginal Ultrasound (TVUS)
- Cervical length ≤25 mm → high risk of preterm labor
- Cervical funneling → increased risk
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
Speculum Exam & Cervical Exam
- Rule out infections (chorioamnionitis, bacterial vaginosis)
- Check for premature rupture of membranes (PROM)
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

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)
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)
Magnesium Sulfate (if <32 weeks)
- Provides fetal neuroprotection, reduces cerebral palsy risk
Antibiotics (if PPROM or GBS unknown)
- Ampicillin + Azithromycin if preterm premature rupture of membranes (PPROM)
- Penicillin if GBS status unknown
Hospital Admission & Monitoring
- Continuous fetal monitoring for distress
- Monitor for chorioamnionitis (maternal fever, tachycardia, uterine tenderness)
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

Complication	Preterm Labor	Threatened Preterm Labor
Neonatal Morbidity	High (if delivery occurs)	Low (if labor does not progress)
Risk of Preterm Birth	High	Low
Neonatal RDS, IVH, NEC	Increased	No increased risk
Long-term Neurodevelopmental Issues	Cerebral palsy, ADHD risk	No increased risk

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