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
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